Toby, therapy techniques can help, but you have to be committed to them. You have to work very hard on, and use the suggestions they make. I don't wholly agree with saying they 'ignore your symptoms', but when they/we are outside the box you are in, we can easily see what is causing your symptoms.
So, (you are a bright lad) its kind of like saying as an analogy, I won't put out the fire but will instead get ride of the smoke by opening the windows. The HA is the root cause of the fear you have, it is the fire.You can work through this bowel cancer fear, then there will be another. Maybe over time you will learn to apply less catastrophic thought processes and learn to reassure yourself and all those other assorted skills which eventually rid yourself of the spiral - I managed it and others have- or maybe you won't and will need to accept that you need to go back for some more professional help.
I have told you everything I know about IBS from my perspective. Your GP thinks you have it based on your symptoms (and pouring through the minutiae of 'but I don't get cramps' doesn't make it less likely that you have IBS, it just means you are luckly enough not to get cramps and have a lesser form). Now it is down to you to reassure yourself.
See, this is the thing, if you don't do therapy, because you don't want to, that doesn't mean you should do nothing. Why not have a look at the book list here and seek out something you can order to help HA? You have to do something Toby and it seems you are choosing self-help.