It is worth remembering that TR61-01 is a CEPT recommendation addressed to national administrations. It is a useful reference document in so far as we can generally assume that signatories have indeed incorporated its recommendations into their national regulations and licences, but TR61-01 is not in itself a regulatory document that applies directly to us as licencees.
It seems to be undisputed that in the USA, the FCC regulations forbid the holder of a US licence from operating in the USA under the terms of a licence issued by another administration. The question on the table is whether the Ofcom regulations in the UK carry the analogous prohibition. Non-existence is hard to prove, but I do not recall ever having seen such a regulation, in my licence or elsewhere. I think therefore it is up to those who assert that such operation is forbidden to cite the regulation that forbids it. I don’t believe anybody has yet done so.
There is of course the general provision in the CEPT agreement that it permits short term use by temporary visitors to a foreign country. If you have recently taken out a UK licence, which presumably involves registering an address, I think it may be hard to argue that you are simultaneously a temporary visitor.
But that is not the same as saying that the use of M/us-call by a UK licence holder is invariably forbidden. For example, a UK licence holder might move abroad. I don’t believe that would of itself revoke their UK licence, but they would no longer be a UK resident. Under such circumstances, they could legitimately claim to be temporary visitors if they came back briefly.
All that being said, exploiting the CEPT licence to gain privilege is not really “in the spirit”, in my personal opinion.