There are several things that we can do to help move it along to the point where they meet up and contionue the story in person if you wish. However there are a number of things I think we need to agree on.
Correct the below if I'm wrong
As I understand it they met when they were 16, he lived in the US for about 2 years before returning to England, which besides being home is also where he went to University. They are both now about 28 years old, its ten years later and have lived 10 years of adult life. During their high school romance they would have shared everything so we would need to agree on what that everything is, their dreams, loves etc. Time does change things but I think there will be new thinsg to be added, the fact that he drinks (more culturally aceptable in the UK perhaps) and she doesn't except for the odd glass of wine. This might of course be a problem but she'll probably wait until they meet to ascertain this.
In terms of the holiday in Florida we should be able to carry that out fairly easily, however when writing in modern day its good to do research and find out about the places to go. For example I actually looked up the flight times as research, ditto the hotel and restaurant (though he hasn't told her which one yet!). Streetview from google is really useful sometimes as one can describe roads half way around the world without actually being there.
In terms of Oxford, its OK having a fictional college there so stick with the one you have chosen, but read up on one that actually exists and base it on that one. For example she might have chosen Somerville, a google search will reveal more info. Once you have the base facts you can then write the story around it.
In terms of the times and date, if its 09.00 am in the US then the UK time wil be five hours ahead, in other words 2pm, So if I send an email at 17.00, its noon when she receives it. Generally he has a couple of time slots, morning and evening, though longer in the evening.
I'm new to writing emails/letters but would suggest that during the mail you ask questions about things, some specific some general (but not "I don't know what to do about this", that way I can write more detailed responses and ask questions in response. At the moment there is an assumption I know a lot about the US where they lived, Ben might - I don't so I can't write about it only reminisce at the moment.
So if you write about something he should know either I need enough info in the post or an ooc comment, but try and leave a hook for me in the letter to respond to, even if just to make up the anecdote.
For example :
"Remember Lucy from school, I bumped into her last week and she has a new bump", gives me lots to talk/ask you about.
"Remember Lucy, I saw her last week" - its too vague, unless you give me something in OOC about her, especially as you may have already formulated a story for her.
In terms of the UK, she has never been so ask some specific questions, please don't make them too generic though. If you need something explained about the visa/etc which Kat should probably understand I suggest we keep that OOC rather than in the emails as she comes across as an idiot, she has been planning this for years so she must know what she is talking about to have gotten into Oxford.
How do you want the story to progress? My view is that the holioday will go well but dont know what your thoughts are about Oxford, maybe she won't enjoy it or maybe she will try and transfer after a term to somewhere closer to Ben?