It will depend on your GP, but I've had quite a bit of help for IBS since coming here, to the point it doesn't affect my life as much as it used to. I'm only 26 but have had symptoms since I was 8, so don't remember life without it, and there were YEARS of constant medical visits. I'm really pleased with the NHS. Sometimes it feels like they want to get through your appointment as quickly as possible, but for the most part it seems they are genuinely trying to help. Of course, the sooner you're feeling better, the less you'll come to the doctor, and that's the aim here when everyone has free access. It's not like they make money off your visit.
About two years ago my boyfriend pestered me to see a dietician because he was concerned about my weight (I've always been tiny). So I did, but rather than investigating my weight, the dietician was able to help me with my IBS. I've been on the FODMAP diet for two years now and it has made a massive difference.
So, all that said, when it comes to GPs here, explain your situation, what you have found works and doesn't work, and even ask to see a specialist if you think it would be of benefit. For stress they can also offer some help, including different therapies, so that may be of use to you as well. All of mine have been really helpful. I've had the odd one or two doctors that weren't great, but I just make sure not to book with them when I call in.
My symptoms are always worse with stress as well, so I feel your pain. When it comes to flying, I have to take Imodium and diazepam to keep calm enough to get on a plane. If you think you'll need something to keep calm for the plane journey, tell your doctor and they should be able to give you something. Heck, lie about it and say you're terrified of flying if you have to - most GPs I tell that are more than sympathetic and don't ask too many questions.