Nigel,
Resello's agreement is governed by the law of the Netherlands. I have no idea whether Resello is violating Dutch law with this policy (and I continue to have no idea whether there is a legal reason for this policy, either...).
With regard to the UK law, out of curiosity, can you point me to where it says citizenship is a protected characteristic?
Under US anti-discrimination laws, there are quite a number of protected classes (including race), but I don't believe citizenship is one of them. Always happy to learn more, though.
Greg