None that I know of imported the terror they had seen in Spain onto the streets of Britain. Indeed, the experience was an eye-opener for most. Orwell famously became a vociferous critic of Stalinism as a result. Many resumed left-wing political activity on return, but by democratic means. One at least - Sir Alfred Sherman - swung so far the other way that he finished as a cheerleader for Margaret Thatcher.
Communism was then as big a bogey as militant Islam is today. Great difficulties were put in the way of the British volunteers for Spain, but it seems that repatriation was easier. It would be surprising if local police were not advised to "keep an eye" on known supporters of either side in the Spanish conflict upon return, but there was nothing like the official demonisation of volunteers for Syria proposed by Mrs May.
To some extent the coalition has brought the difficulties on its own head. If it had not been so outspoken in favour of the forces opposing President Assad, making the anti-Alawite revolution respectable, maybe fewer Sunnis from this country would have been encouraged to join.