To answer some of the questions, no, there are no postal restrictions between Spain and the UK for sending the gel.
I always put a tube of gel in my hand luggage in case my suitcase gets lost but it is within the limit (80 grams) i.e. under 100 gram hand luggage restriction.
Callisto, you don't need to contact the pharmacy in advance of travel but make sure you order it on day 1 of your holiday so that it is ready in time. They usually have it in for the next day - interestingly, the gel is made in Barcelona.
As for the NHS and their prescription guidelines, they are all over the place, some GPs will prescribe it and others won't. I gave up trying to get it on the NHS years ago and just do my own thing. Life's too short!