Let's see...
Tell her to bring any food item she really can't live without for awhile, as shipping is highway robbery. And that eventually she will get used to not having the American foods there.
That you can buy Reeses pb cups, tootsie rolls, fruit loops, and libby's tinned pumpkin here in the UK, so life is perfectly liveable. 
Don't bother bringing chocolate to the UK as it's much much better here in the UK. You will never look at a Hershey's bar the same way again. 
Closet space? I vaguely recall having closet space in the states. Haven't had any in ... oh... ten years.
You make do.
If she has any particular clothing items that she likes (I bought a few pairs of jeans of my favourite kind and brought them along - and 10 years later, I still have 2 pairs I'm still wearing) then buy a few extra and pop them in her suitcase to bring along.
Consider buying a small car to start with - because practically having a heart attack every time you drive by a bus on a narrow rural road is no fun. I know. I've been there. 