You say ratingS. If this means you are also getting your FAA IR and you'll be flying a G-reg plane in IMC in the UK then you should expect a lot of differences. If you're in an N-reg plane you'll be able to fly in Britain's Class A airways and the differences will be much less great. If you're flying a G-reg plane you'll not be able to fly airways at all, you'll have to "negotiate" radar cover and will often find it unavailable even in very busy bits of airspace, you'll often have to "freecall" one station after another changing squawk as you do so, you cannot routinely expect to be able to do instrument approaches in VMC without having booked "a slot" in advance, you'll have to pay for most instrument approaches over-and-above regular landing fees, and more.