Thank you everyone for all your information

Your passport was stamped by Spanish border control; nothing to do with the UK Border Force.

Was NOT Spanish Border Control.Stood at a section labelled BRITISH Border Control,manned by all British personnel,spoke English,even had a short conversation with that person about where she had lived in UK.Travelled to many places in Europe during my 4 decades living in Spain during the time we were in EU & never had a stamp before,left whilst we were still in EU so cannot see the reason for that stamp.

This stamp of yours, does it have a picture of a plane with an E surrounded by stars in the top left corner?

We just had a week in Barcelona and the entry stamp was as you describe plus an arrow pointing to the right. And on leaving we went through Spanish border control where our passports were stamped again but in the bottom left hand corner the new stamp has a left pointing arrow showing it as an exit stamp.

As is usual we didn’t encounter British border control until we arrived in Britain. When we have arrived on board a cruise ship then sometimes British border control put officers on board when we arrive to speed up the process.

"British Border Control" is not a term/ description used by the UK's Border Control agency.

I can find no record in the UK's Border Control agency's web pages, or any third party pages, about there being UK Border Control agency locations in Spain, or indeed anywhere else outside the UK.

There is no reason why the Spanish government can't hire British citizens to staff a desk that is designed to serve primarily English-speaking British citizens, and indeed there are good reasons for the people staffing the British desk being native English speakers. The British staff operating what appears to be a EU/Spanish exit control checkpoint that stamped your passport, may already have been in Spain pre-Brexit, and/or have a Spanish spouse that would qualify them for a visa. And there is no reason why the Spanish government could not issue a British citizen a visa to work for them in Spain, though given how many British citizens already live in Spain, I suspect that it relatively unlikely.