Well, I've re-thought this given that it's between 5 and 6 hours each way; with my colitis symptoms making that high risk I've decided to go with another destination that's a bit closer.
In fact, I've been thinking of taking the bike down to Lymington for a while now so I can spend a day or two enjoying riding through the New Forest (as well as taking advantage of Lymington itself) It's a 3 hour journey, which seems a bit more doable, and I'm now fully booked here for the last weekend in April. I can cancel at no cost up to the day before departure too, which means if I'm having any kind of flare-up I just won't go (although I'm hoping that's not going to be an issue, of course).
Just to add, I'll be at the Sammy Miller Motorcycle Museum on the morning of Saturday 29th, so if anyone fancies meeting there and then going for a ride through the New Forest let me know by PM.