A mare needs to be at her foaling destination and settled in at least one month before her foaling date, due to her being able to gain immunity to any 'germs/etc' and thus be able to pass this on to her new foal. Remembering that a mare can foal, say, two weeks before her actual due date, would mean that she should be at her new home at least 6 weeks before that date.
I know that TB mares regularly travel and fly to outside studs to foal there and be covered before retuning home, and a lot of these do work to a 4 week 'travel time' programme. However most of those horse are well used to regular travel and are with travel companies (or their owners) who are well used to caring for them.
Some of the questions I would want to ask before travelling her if she were mine would be - what kind of transport? Crossing the channel by sea or by the tunnel? Is she a seasoned traveller? Is she generally a calm, unstressed sort of mare?
Transport - would she have her own compartment? Would long enough stops be made for her to rest her legs from the constant movement/need to balance with her extra weight? On stops would she be untied and given enough space and the room and time to lay down if she needed to, plus enough bedding to be comfortable (our little stallion had a 2 hour long sleep laying down in a huge deep bed of straw, on one of our stops when collecting him from the airport, he was just shattered!
Would she be in a compartment between two large horses who could perhaps reach over to annoy her during the journey?
If crossing by sea, will they wait for a calm crossing, if by tunnel, what precautions have they taken in case of an electrical breakdown and getting stuck for hours in the tunnel?
Once in the UK how much further does she have to travel before reaching her new home?
Will her 'stall' contain a good amount of some sort of bedding (good amount, not just a scattering), there is nothing worse than an unshod horse trying to get a firm footing on rubber matting that is covered with fresh droppings and pee!
After considering everything I have written, I have to say that if she were mine, I would leave her where she is until the foal is at least a month old and then organise specialist transport to bring them both home - someone happy to give them plenty of space, loads of bedding so the foal can lay down and sleep, and the time for long stops to give them the rest they will require.
Hope this helps - good luck!
Anna