Renfe reservations in advance from UK?

They will be available whenever RENFE feels like it.

Here's an overview of the known ways to book reservations for domestic Spanish trains:

And-as this very long route is also the most asked for and also the most refused/non available (somehow my cynic nature tells me RENFE does not want to have too many passes on it) - and this you could also have found by simply just looking a tad further: AVE Bcn-Mad run hourly and Mad-Mlg about 2-hourly (or more often with new change in Cordoba)-this will lengthen trip and double cost, as 2 RES needed, but makes access far more easy.

The train from Bcn has 2 sets-split at Cord-the other will go to Sevilla or Granada-maybe that one may have seats left.