Seen in woodland in southern Spain last week
Those antennae identify it as a cockchafer, Melolontha, of which there are three species in Europe.
Since Melolontha melolontha is absent from Spain, and Melolontha pectoralis is very rare and occurs only in south-western Germany, we can be pretty sure yours is Melolontha hippocastani, the forest cockchafer.
Note: "Male cockchafers have seven "leaves" on their antennae, whereas the females have only six."
Information sourced mainly from:
Update: Since Florin has pointed out (below) that M. papposa also occurs in Spain, my info on there being three species of Melolontha in Europe is clearly wrong; my apologies.
This is definitely other species than the British one (M. melolontha). Males have larger antennae than females, but this one has ridiculously large ones. It could be a chafer that lives in Spain, Melolontha papposa. Here is an example.
Good info, Florin.
And those are definitely different species (not synonyms).
There are some useful photos of both here:
From a little research just now, it seems there may be no/few macroscopic characters that can be used to differentiate those species; dissection could be necessary.
Thanks, Mike. I prefer H. papposa for this specimen, unless it's a species other than M. hippocastani or M. melolontha. M. hippocastani seems to have two colour forms, one with chestnut-red pronotum and reddish-brown legs, the other with black legs and pronotum. Wikipedia have an image here. But of course, I may be wrong. I've only seen the common species M melolontha.
Thanks for the further discussion. I cannot comment, but I trust your judgment on this matter more than mine.