Actually, as the callsign is M*1EYP, where * could be one of several things (including nothing at all), then would it really matter what you put there if overseas? Your UK callsign would still be uniquely identified, and the * is, effectively, ignored, other than for sake of interest.
A discussion along similar lines came up about whether it is EA/M1EYP or EA8/M1EYP when operating from the Canary Islands. The only requirement is for the EA/, but it is not prohibited to add the extra geographical information - EA5/, EA8/ etc.
Other countries might be different - though I doubt it. Certainly, with Spain (which I looked into in detail), Andy in Tenerife would be EA/M0FMF, but there would be no objection to EA8/M0FMF. Similarly, I imagine there would be no issue with EA/MM0FMF or EA8/MM0FMF as the UK call is uniquely identified in each case. The RSL is merely a piece of optional supplementary helpful information that does not alter the unique UK callsign. In this regard it is similar to /P or /M. Rarely mandatory, but potentially helpful optional information.
I have just had a good read through the RSGB, Ofcom, CEPT and EA official documents, and see nothing to suggest such practice is prohibited. Certainly the minimum requirement is only the UK call without RSL, but I’m not convinced that it is any more prohibited to optionally add it than it is for /M, /P etc.