I'm British myself but I've been living in the US west-coast for the past 7.5 years because I get paid 2-3x more here for the exact same work (heck, even 4x if you don't count London).
The reasons for the disparity are as complex as they are legion - but I believe the size of the market you can sell to matters the most - and with the UK out of the EU the size of the effective market it can realistically sell to has shrunk considerably, so I don't see things getting better at all for the UK eng sector.
...secondarily, the UK is having the same problem the US is having with boomer-generation people still working and occupying senior positions... and housing... which limits opportunities for nominal upward mobility in younger professionals, which in-turn suppresses total-comp. This is especially a problem given the UK's entrenched business culture which I'm not personally a fan of.