When did the UK separate from Europe?

askhistory said no. Also, sorry, no idea what flair this is under.

I'm trying to look it up but it's says it happened 450,000 years ago when a lake burst.

10,000 years ago the land bridge flooded.

Then it says at 45 bc it still wasn't separate from Europe because the English Channel didn't exist.

Can anyone explain it, please? I also wanna know when mankind was travelling to or from the UK. It says Julius Ceaser was in 54 bc.

Many thanks!