Lindy,  you can get the REMA protocols and premedicate for your flight.  This is what I do when I fly to the States from Brazil, so I know the stress of a long flight.  By premedicating, it will leave you less concerned about reacting since you should be properly prepared.   However, reserve a wheelchair.  I saw the size of Heathrow these days and there's just NO WAY to walk through those terminals!   You don't need to be wasting your energy with all that walking before the flight begins and you won't have the energy to use after the flight ends, so a wheel chair is a life saver!  It's other benefit is that when you are flying internationally or nationally due to either the immigration and customs lines or just the fuss with the security section, being in a wheelchair gives you VIP treatment the entire way through!!!  

So, ,don't feel like you're a 2nd class citizen, but take full advantage of protecting yourself and saving you and whoever is with you a huge amount of stress having to deal with the airport mess!!  I tell you, my family was SO pleased that I was in a wheelchair when I went to Ireland 3 years ago for not only did we bypass ALL the lines, but going through customs was an absolute breeze for all 5 of us!!!  And my husband really appreciated it because he didn't have to worry about me and what all that stress would do to me!   Yet, it ended up that it saved my life because on the way back home I went into anaphylaxis that was held in check by the protocol meds.  If I'd had to walk it, I'd have gone into a full blown attack!

So, think about it Lindy!