I am trying to find out if since the 2012 London Chess classic, it is the strongest ever tournament help in Britain.
Sorry, it's a little unclear what your question is. Are you asking whether the 2012 London Classic was the strongest tournament ever held in Britain at the time or are you asking if it has been displaced from the top spot by a more recent one? I've no idea, but their website might say.
Originally posted by @plopzilla I am trying to find out if since the 2012 London Chess classic, it is the strongest ever tournament help in Britain.
Absolutely or relatively? Remember that Elo-ratings drift upwards, so comparisons are deceptive. For example, compared to the rest of the chess world then, isn't Hastings 1895 stronger than many modern tournaments?
I've used the website chessmetrics.com (which seems to have a decent methodology, though it'd be worth checking for yourself if this question is particularly important) before to compare historical ratings, if you are at all interested in considering inflation. In any case, among their strongest tournaments
is Nottingham 1936:
which seems like a contender at the minimum.