Comment by michaelt

More specifically in the UK there are a few ways employees can be dismissed.

You can be dismissed when you have done something wrong, in which case there's no notice period but the employer has to be able to show they've followed certain rules.

You can be dismissed when you haven't done anything wrong, in which case you either get several months notice or several months pay ('in lieu of notice') or a 'voluntary settlement agreement' (more pay, negotiable terms) all subject to slightly different rules.

So a US employer can cut a UK employee's computer system access the same day, it just costs a bit.