Passei as últimas três horas pesquisando esse fórum para uma correção. No entanto, e embora pareça haver problemas semelhantes, todas as correções foram inúteis.
Ign:27 http://gb.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
Err:49 http://gb.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
404 Not Found
Ign:50 http://gb.archive.ubuntu.com/ubuntu xenial-updates/main i386 Packages
Ign:51 http://gb.archive.ubuntu.com/ubuntu xenial-updates/main all Packages
Ign:52 http://gb.archive.ubuntu.com/ubuntu xenial-updates/main Translation-en_GB
Ign:53 http://gb.archive.ubuntu.com/ubuntu xenial-updates/main Translation-en
Ign:54 http://gb.archive.ubuntu.com/ubuntu xenial-updates/restricted amd64 Packages
Ign:55 http://gb.archive.ubuntu.com/ubuntu xenial-updates/restricted i386 Packages
Ign:56 http://gb.archive.ubuntu.com/ubuntu xenial-updates/restricted all Packages
Ign:57 http://gb.archive.ubuntu.com/ubuntu xenial-updates/restricted Translation-en_GB
Ign:58 http://gb.archive.ubuntu.com/ubuntu xenial-updates/restricted Translation-en
Ign:59 http://gb.archive.ubuntu.com/ubuntu xenial-updates/universe amd64 Packages
Err:69 http://gb.archive.ubuntu.com/ubuntu xenial-backports/main amd64 Packages
404 Not Found
Ign:70 http://gb.archive.ubuntu.com/ubuntu xenial-backports/main i386 Packages
Ign:71 http://gb.archive.ubuntu.com/ubuntu xenial-backports/main all Packages
Ign:72 http://gb.archive.ubuntu.com/ubuntu xenial-backports/main Translation-en_GB
Ign:73 http://gb.archive.ubuntu.com/ubuntu xenial-backports/main Translation-en
Ign:74 http://gb.archive.ubuntu.com/ubuntu xenial-backports/restricted amd64 Packages
Ign:75 http://gb.archive.ubuntu.com/ubuntu xenial-backports/restricted i386 Packages
Ign:76 http://gb.archive.ubuntu.com/ubuntu xenial-backports/restricted all Packages
Ign:77 http://gb.archive.ubuntu.com/ubuntu xenial-backports/restricted Translation-en_GB
Ign:78 http://gb.archive.ubuntu.com/ubuntu xenial-backports/restricted Translation-en
Ign:79 http://gb.archive.ubuntu.com/ubuntu xenial-backports/universe amd64 Packages
Reading package lists... Done
W: The repository 'http://gb.archive.ubuntu.com/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://gb.archive.ubuntu.com/ubuntu xenial-updates Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://gb.archive.ubuntu.com/ubuntu xenial-backports Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://security.ubuntu.com/ubuntu xenial-security Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/xenial-updates/main/binary-amd64/Packages 404 Not Found
E: Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/xenial-backports/main/binary-amd64/Packages 404 Not Found
E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/xenial-security/main/binary-amd64/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
Este é o meu arquivo /etc/apt/sources.list :
# deb cdrom:[Ubuntu-Server 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801)]/ xenial main restricted
# deb cdrom:[Ubuntu-Server 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801)]/ xenial main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://gb.archive.ubuntu.com/ubuntu/ xenial main restricted
# deb-src http://gb.archive.ubuntu.com/ubuntu/ xenial main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://gb.archive.ubuntu.com/ubuntu/ xenial-updates main restricted
# deb-src http://gb.archive.ubuntu.com/ubuntu/ xenial-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://gb.archive.ubuntu.com/ubuntu/ xenial universe
# deb-src http://gb.archive.ubuntu.com/ubuntu/ xenial universe
deb http://gb.archive.ubuntu.com/ubuntu/ xenial-updates universe
# deb-src http://gb.archive.ubuntu.com/ubuntu/ xenial-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://gb.archive.ubuntu.com/ubuntu/ xenial multiverse
# deb-src http://gb.archive.ubuntu.com/ubuntu/ xenial multiverse
deb http://gb.archive.ubuntu.com/ubuntu/ xenial-updates multiverse
# deb-src http://gb.archive.ubuntu.com/ubuntu/ xenial-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://gb.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
# deb-src http://gb.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu xenial partner
# deb-src http://archive.canonical.com/ubuntu xenial partner
deb http://security.ubuntu.com/ubuntu xenial-security main restricted
# deb-src http://security.ubuntu.com/ubuntu xenial-security main restricted
deb http://security.ubuntu.com/ubuntu xenial-security universe
# deb-src http://security.ubuntu.com/ubuntu xenial-security universe
deb http://security.ubuntu.com/ubuntu xenial-security multiverse
# deb-src http://security.ubuntu.com/ubuntu xenial-security multiverse
Como resolvo esse problema?