Driving license

I exchanged my U.K. licence for a Spanish one - admittedly about 15 years ago - and all I had to do was to take some basic physical reaction test and a sight test.  There was no declaration of any kind involved.  Hopefully someone on this forum with more recent experience of doing this will be able to give you up-to-date information in case anything has changed.

A pal exchanged his about this time last year without any sort of extra documentation but the need for something extra could be one of the Brexit changes I suppose.

The only people that could give you such a certificate would be DVLA but I think that's what the Spanish traffic people do - it's always the excuse that gestores use for the time it takes to get a licence exchanged!