To be brutally honest, Burton are probably one of the worst places to go for suits, personally I haven't even touched one of their suits but their modelling online is absolutely diabolical, I have never seen worse fitting suits and I'm not entirely sure that they've just given the models the size up any more.
Sizing inconsistencies tend to arise from cheap materials being made into cheap clothes for a tiny amount of money on a mass scale and with regards to cheap suiting, on terrible original patterns. It just won't ever be as good. If you've ever wondered why all these young guys that go to University graduations or Sixth form balls etc look so bloody awful, its because they've bought a cheap suit. It is just something we have to deal with sadly.
Part of your problem is perhaps that you are looking for an off the peg suit that fits you even reasonably everywhere. Unless you are very lucky, that just isn't going to happen, no one is a brands idea of a standard size.
My suggestion, going on the what you've said above, is to go for the Topman slim fit, the jacket shouldn't come up any higher than the bottom of the trouser zip and it should completely cover your arse; I know Topman jackets tend to be slightly shorter than most but it's still worth bearing in mind. The trouser legs riding up when you stand up is just a factor of them being a slim fitting trouser, you leg expands when you sit, all you have to do is push them back down, they might even slide down of their own accord.
Your only other option is tailoring I'm afraid, find the suit that fits the best and get it altered, basic alterations (tapering, trouser length, jacket waist, sleeve length) shouldn't cost more than £40.
It's frustrating but it's just a fact of life. If you intended to buy two suits at the £150 each mark stump up another £85 and you can have This suit:
Youngs Ink Sb 1 Button Peak - REISS
Which I can assure you looks a lot better in real life, with the trouser alterations included. If you do have a local Reiss you could also check out their mixer suits, which are essentially separate blazers and trousers with slightly more casual detailing or colouring which come to about £345 - £350.
Its another alternative to consider, they will last a lot longer and look a lot, lot better.