The first thing that has to be assumed in anything remotely credible is that there is no significant navy defending the UK. This is somewhat unlikely because of the Royal Navy - until post-WW2 it was aimed to be the equal first in power in the world, and anyone who managed to invade the UK despite that would need a powerful navy of their own (simply to overmatch the RN) or perhaps enough naval air power to be the equivalent of a couple of dozen aircraft carriers.
So there's a large navy defending the British Isles. What this in turn means is that you need to bring an
overwhelming navy of your own - in the 1940s it is possible with UnRep to operate a large navy a couple of thousand miles from a base, but you'll probably need some convenient large base to operate from (Iceland? The Azores? Both are quite a long way off and Iceland is quite suboptimal).
But let's assume you do it. Now you need to get ashore.
The best place if you're coming from the west is probably Ireland - it exposes you to less in the way of air attacks and small-naval sorties which would otherwise harm your supply line (either up the channel, into the Irish Sea or past Scotland) from the many excellent UK ports. But you'll need to get over the Irish beaches, and also capture or bring along your harbour.
Only problem... the people who OTL invented the portable harbour are the people you're invading, and that couldn't cross oceans - while there's only a couple of really good ports on the Irish west coast (Limerick looks like the best).
Perhaps you should try landing on the west coast of Devon instead, but that's also bereft of large ports (I mean... Bude...) and the best port with that side of the UK as the approach is probably Bristol.
Which is to say, a port in the Severn. With one of the largest tidal ranges in the world.
It ain't easy.
I don't think they could resist an attack like that, and the US was always better at submarine warfare than the Germans, so a blockade could actually be maintained.
Honestly, that one I'm not sure of - the US was better
against the Japanese than the Germans were
against the British, once the torpedo problems were worked out. It's going to be hard for the US to blockade British-French or British-Dutch trade, as well...