Dear Blue, I understand your fear of traveling at this stage of your illness. Before I went away for the week I was also anxious. I took meds for all sorts of possibilities and references to doctors - important(your travel insurance should cover medical) - to take care of any eventuality.
Things were ok 'though. The lupus and the ME played up a little, but got sorted out with rest and a few extra pain meds.
Be careful but don't worry too much. The minute you feel any warning bells, take a step back and assess the situation.
Make sure you are physically comfortable - lots of warm clothes . . . waterproof stuff, umbrellas . . . gloves ... woolly caps ... bad weather in London now.
Once you are set up with everything you can think of that you may possibly need, go out and have a blast. The excitement and the adrenaline will keep you going so don't worry.
Have a great break - you deserve it.