The other reason is more mundane. There's been a lot of political incentive to reduce immigration for a long time, which means adding arbitrary friction to increase the effort of applying and decrease the number of successful applicants.

Whether this is _effective_ is a different question, but certainly it's gotten a lot harder in recent decades, even pre-Brexit.

That explanation doesn’t seem to jive with the fact that post-COVID the UK has accepted millions of immigrants

it does, if the alternative would have been "more millions"

That millions were accepted says nothing about the process having changed for the worse.

lol, that’s extra funny considering that the boat people can just appear and receive not only the de facto right to remain, but get free hotel rooms for years. Meanwhile productive people with useful skills are tortured with red tape.