Britain took 4 years to negotiate and prepare for the new trading arrangement with the EU. It wasn't all smooth, but they did things like massively expand customs facilities at sea and airports to allow for the new inspections, train up inspectors for animal and plant imports. It gave time for people to move, companies to move their HQ, set up new warehouses, etc. Both sides were clear that the arrangement wouldn't be changed back with a change of government — the EU had no patience for that, opposition parties in the UK said they wouldn't aim for it either.

The USA hasn't done any of that, so the situation at this stage isn't the same.

Weeks after Brexit happened, when Covid started, EU governments quickly moved their focus to the pandemic. Since then, they've continued to tell Britain they aren't interested in reopening discussions so soon after new treaties were signed.

That's changed slightly with Russia/Ukraine/Trump, as the UK has positioned itself carefully between the USA and the rest of NATO.

All that time though, it remains a topic of debate in Britain since they haven't seen any of the promised benefits of Brexit. Some things have got worse, but I'd say it's generally a problem of stagnation, compared to improvements in comparable neighbouring countries.