Yeah. And until you've got your medical and SEP sorted out, you don't have a valid license, and only valid licenses can be transferred. So
AFAIK there's no way to transfer whatever you have now beforehand...
Anyway, your SEP renewal is an exam that needs to be done with an examiner, not with an instructor. Examiners need to be known to the relevant *AA beforehand, and in a lot of cases examiners need to ask permission for an exam beforehand as well. Again, from the relevant *AA. It's not the *standards* that are different, but it's the *procedures* (forms and other paperwork) that are different.
EASA has produced a document for examiners where each countries local procedures, forms and approval process are described. Any examiner in EASA land should have heard of this document and should have the latest version bookmarked. And they should, before each exam, check for and perform any preliminary actions. So in theory, all this should not be your problem. But having said that, if this is the first time your examiner performs a "foreign" exam like this, you may want to give him some help...
https://www.easa.europa.eu/system/fi...2017_ORA_0.pdf
Similarly, your AME should know the differences in local procedures as well - again, it's not that the medical *standards* vary, but it's the *procedures*. Before your medical exam, make sure you detail your situation to the AME and they should be able to arrange the paperwork around your medical so that it is acceptable to both the DGAC and the CAA. That will facilitate an easy migration later on.
By the way, is there any particular reason that you want to transfer stuff over to the UK CAA? You don't *have to* transfer your license & stuff to the *AA of your country of residence: It's completely optional. I live in the Netherlands but my license is administered by the UK CAA. Not that I'm completely happy with the UK CAA, but the Dutch KIWA is not so much better that I'd want to move across. And a license transfer is not entirely free...