Well we had the London Monarchs who played at wembley for a while in that crappy NFL spin off league (WLAF), and the first eason attendances were pretty good iirc, but as time went on i think they dropped as people realised it was a second rate league, just checked attendances were between 35 and 50 thousand in the first season, then pretty much tailed off a lot as the product wasn;t all that great
As for a proper pro team in the UK, logisitically it would be pretty shitty tbh, and the time difference would be too much, pretty much would never get a home game on MNF, as it's about 1.30am UK time that it starts, and stuff like that, also for some of us brits who already like NFL we have our own teams we follow, and yeah going to some games would be cool, but it's unlikely that my team would relocate here and it becomes less of a draw to go and see a team several times a year that you don't actually support, once a bear always a bear