Hi there Emily
Officially, you do have to be within two hours of the airport at which you're based. This is more a CAA regulation than an airline regualation, as it's suggested that having to travel any more than two hours to your base would mean you are too tired to perform effectively in the event of an emergency.
This rule is, however, largely ignored.
Commuting to Eurofleet at either Heathrow or at Gatwick is difficult. I commute to LongHaul and spend my flights from Aberdeen chatting away to some of the crew members. I do come across a few who commute who say it's OK, but as you can often get there-and-back trips on the same day, you might find yourself commuting two or three times a week in each direction (really don't know enough about the EF rosters).
I know one girl who commutes to LGWEF from BCN and she says she manages just fine.
If I were you, I'd give it a go. You'll kick yourself down the line if you didn't try it for a while.
Good luck to you.