What you see posted is the salary you can get when you are hired. They don't match an American salary.
If you get an offer from the US while teaching in the UK, they can promote you to a higher rank which would imply a higher salary. The better places might even give you a bit of a retention bonus, but it is unlikely to go into base salary, i.e. it will just be temporary.
Here's a typical scale:
Senior Lecturer £48-55
Professor £60-110, but as mentioned, that's entirely up to negotiation and can go up to £150. I do know though that many profs in polisci have salaries in the 60s or 70s at most.
Remember that these are automatic increases until you hit the Reader level, so no arbitrariness with the merit pay like at many US universities, but after that it's all market based.
British universities are also under the strange impression that full professors make the most important contributions to the discipline, i.e. blokes in their 50s and 60s.
A decent UK job is a very good consolation prize if you've struck out on the US market, surely beats VAP, adjuncting, or taking anything that's not an R1.