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Where are you planning to end up at the time you might come to sell gold? 
Depends if you eventually want to end up back in the UK and sell in the UK or whether you plan to stay in Spain forever... Do you have any local coin or bullion shops? Ask them what are their most liquid items perhaps?
Some areas of Spain still use a wealth tax at various rates which could be a consideration depending on your other assets too (impuesto de patrimonio) 

Many bullion dealers around the world stock coins and bars from all the other countries, so no need to necessarily buy from a dealer in a different country. Though many dealers do of course ship worldwide also.
I am not sure what dealers there are in Spain but worth checking to see what coins they stock as they likely stock U.K. coins at similar prices to what you can get in the U.K. anyway?

Many people in the U.K. like the Britannia and Sovereign coins as well as other legal tender U.K. gold coins because they are CGT (capital gains tax) free in the U.K. though of course that doesn’t apply in Spain. I am not sure if there are any other similar type of benefits for owning Spanish gold in Spain. 

I believe though may be mistaken that bullion/investment gold imported into the EU is tax free. That’s also the case for importing gold into the U.K. although it is relatively common for customs to make an error, charge import taxes etc, which then need to be claimed back.

You can also look at dealers such as @EuropeanMint and @EuropaBullion who are in Europe.