Scenes of the last battle were filmed in Pescola (Valencia).
Also appears a castle located in Manzanares el Real (Madrid).
I'm pretty sure, Bronston doesn't filmed any part in England.

I was also surprised to read that information regarding the locations, and the last time I saw the film last Christmas I kept watching carefully for the supposed British location near Bamburgh Castle. As I far as I can tell, it may be one of the night shots in which we see the silhouette of the castle in the background. But because is it twilight and the castle stands so far away it is almost impossible to tell for sure.

what scene was filmed in Italy?

I wonder if this might be getting confused with 'Becket', the Richard Burton epic made a couple of years after El Cid. Becket did indeed have scenes filmed at Bamburgh Castle.

Two Theories.
I suspect these scenes might have been done for the same reason they shot a scene in Italyto qualify for tax advantages under British..and Italian..tax laws. Shooting a single scene in Italy technically qualified it as a Italian coproduction,and maybe the same thing went for the UK.
Or,since the film was edited in London,they might have decided a few "phonebooth" shots for the Battle of Valencia were needed,and since there were close up of a few men fighting,it was cheasper to ship in a couple of dozen costumes from Spain and shoot them on a beach in England.This is not unusual.

The only information that I have is that some sequences were shot on the sands below Bamburgh Castle. Which sequences those actually were, I've no idea and why they were shot there is even more of a mystery.
I've carefully examined all the beach scenes in El Cid, but I've not been able to identify which ones could have been filmed at this location.