My understanding of the EASA regs is that the national aviation authority which issues your licences / ratings must be the same one that holds your medical. So you can't get a DGAC-issued EASA medical (as and when they get round to issuing them) and then train for a licence / rating at a UK-CAA registered ATO.
Or rather you can, but you would then have to have your medical records translated into English, rubber-stamped by the DGAC and forwarded to the UK CAA. Recent experience has shown this to be a rather expensive exercise, and given that an EASA medical is the same across all member states (as and when the rest of Europe adopts EASA Part-MED) you wouldn't gain anything in terms of less stringent medical requirements or longer periods between checks.