There are European states that will not allow UK TRE's to sign revalidations / renewals and as mentioned above there are additional forms to fill in (SRG1119). This isn't the only reason, if a non UK rating expires on say the 10th of the month, then that is the date it expires. If a UK rating expires on the 10th of the month, we can extend the validity to the end of the month, the aims software which tracks rating validity will assume that all ratings expire at the end of the month and potentially someone flying with an invalid rating. I have seen this before on a UK licence where the rating had not been extended for some reason and the person flew illegally.
Regards
MF