Lucky Louise! Long haul is just exactly that -- and it's an even longer haul when you are dealing with a chronic condition! My secret is booking a seat in first class ... lie flat bed etc. ROFL.... kidding... I always fly back-of-the-bus economy! And what has worked for me many many times on flights of more than 10 hours, is to take a sleeping medication. If I take a sleeping pill, I have much less pain and stiffness that comes from sitting in one position for an extended period of time. I've tried over-the-counter meds, but they don't even come close to the effectiveness of a prescription sleeping pill. Trick is, you need one that doesn't leave you with a hangover. Discuss it with your doctor, and I'm sure s/he will come up with something that will work for you. And before you take it on the plane, try it at least once at home. (Oh, and when you tell your friends you are doing this, they will say "oh, just have a stiff drink, oh just use seasickness pills, just take this herbal stuff ... trust me, been there, tried all that, and you will get off the plane feeling like a bag of dirt.)
Also go through the "what if" scenarios with your doctor. Like, what if, despite Simponi, you get a flare or pain when you are in the UK? Should you go "armed" with a course of prednisone, to use if necessary? A pain medication? For sure, you can get those in the UK as needed, but you might be spending precious days of your holiday trying to ride out whatever-it-is, and hoping it will go away on its own, and then more holiday time waiting to see a doctor if it doesn't. Better to go prepared!
And yes, take your meds in your carry-on baggage. Except for a 3 or 4 day reserve supply in your checked luggage, along with prescribing information. That way, if your carry on is lost or stolen, you will have a few days to replace your medications. My pharmacist prints me two copies of a "patient summary" for each trip: one for my purse, one for my suitcase.
Finally, ask for wheelchair assistance at the airport. Even though you are fine walking, queuing/standing for this and for that really takes it out of you. When you get to the UK, you don't want to spend the first quarter of your holiday recovering from the journey.
If I think of anything else, I'll add it. What a great adventure for you! And what a great thing to look forward to!
Happy travels, and yes, we want to see pictures. ;-)
Seenie