of course it is valid. But for licence issue you need the medical issued by the same authority as the licence you are applying for, so, as you have almost a year to go between your initial medical and licence issue, you merely need a UK renewal when the time comes.
If a Spanish medical was not valid, how would Iberia, etc fly in UK airspace? same goes for an FAA medical and United or Delta - of course they are valid in the UK. - Just not for UK licence issue.
Anyway, after licence issue, you will be able to renew in any JAR country, but when the time comes to unfreeze your ATPL, you will have to get a UK renewal before again.
hope this explains it. IP