Hi Matty, I have attached some info which explains the Driving In Spain/EU developments after Brexit. This booklet has been provided to all Gibraltarian UK citizens by our government and covers a whole aspect of no deal scenarios. Please bear in mind that Gibraltar was not included in the UK/EU deal. The last paragraph, though, does apply to all UK nationals, even though it is directed to Gibraltar driving licences. The tricky bit here is the residency. If you purchase a Spanish plated bike you will need to have, by law, a Spanish log book. This means that you would need to stipulate your address in Spain which by default renders your UK drivers licence invalid, unless you can prove that you have not been in the EU, for less than 180 days. That could be dodgy.
IMO I think you would be better off getting a UK plated bike, driving it down and keeping it in Spain. In that case, you could drive around and if stopped by the friendly Guardia Civil you can always show the your UK drivers licence ,which is miles better than having to exchange your UK drivers licence for a Spanish one simply because you are driving around with a Spanish plate. You will not get away with driving a Spanish plated vehicle in your name with a UK drivers licence.
Regarding the exchanging of Drivers Licences, it is presumably just a straight swap.