Real GDP per capita growth for US vs UK was almost identical until 2008. The last 3 years have been terrible for the UK, but if you're looking for the start of UK's stagnation you have to go much farther back than Brexit.
See:
https://www.euronews.com/business/2024/01/31/brexit-four-yea...
which is a source that is not going to be biased towards Brexit success, and instead against it (not that there was any success in it, but still).