[PaulRobinson]

That would be a disposal and requires an amendment to the law (specifically the British Museum Act 1963, which I've quoted in a reply to another comment in this thread).

"Everyone involved understands" is not sufficient guarantee to either the Museum or the Greek authorities that the law has been changed. It either must be a temporary loan - and ideally with artefacts of similar value being lent the other way to make clear that the loan is "de-risked" - or it can't happen.

I can't see that change in law happening any time soon.

[megapolitics]

I don’t believe the courts would rule such a loan to be a disposal given the trend towards judicial activism in the UK.