Search the forums using “Spain reservations” for the answer, there are various options (via Swiss or Germans railways for example) but none of them are very convenient unfortunately!!

Thanks for your reply- I have read through those already but will I be able to use the Swiss or German service from the UK?

Yes, there is a phone number for the German service but it says that they have to post you out the paper reservation, so there might not be time, depending when you’re going in July. Why not give them a call and ask how long it takes to arrive by post?

The known ways to book Spanish domestic trains are in this post.

The known ways to book TGV/AVEs between France and Spain are in this post.