Como vocês aconselharam, eu enviei um email para o endereço de ajuda da loja online da Microsoft. Eu recebi a seguinte resposta:
Dear Mr North,
Thank you for your e-mail.
It is no problem at all for you to use an Office Home and Student edition in the context described by you. For further inquiries about Office products please click on this link:
http://emea.microsoftstore.com/UK/
Please feel free to contact us if you should have any further questions. We are happy to help.
Kind Regards
Manfred Löchtermann
Então parece que está tudo bem, afinal.
Obrigado pela sua ajuda.