If the endorsements are not carried over and you retain your UK license, but then drive a vehicle in Spain that you couldn`t on a Spanish License I`m not totally sure but my understanding is that you could be prosecuted.
I think driving a UK license in Spain when it has been registered with Traffico only entitles the holder to drive a car and moto upto 75cc, if you drive outside that class, you need a Spanish license.
For Buses, all drivers need to pass there Spanish driving course, Spain does not recognise other countries qualifications for public transport.