I think going to somewhere like Millets or Black's is a sensible starting point, because you need to work out what fits your feet. These companies tend to reuse their lasts, so once you've found one that works for you it is feasible to buy online where last year's model (often just last year's colours) can be had at significant discounts at the right time of year.
The problem with fabric boots and membranes is that they tend not to last. As the foot and the shoe moves, it's almost inevitable that rubbing occurs, which will eventually work through the membrane. Once that happens, depending on where the whole is, the boot will no longer be waterproof (in particular when it comes to wet grass, deeper puddles, or small streams). This doesn't affect all models, but I don't think the information about which ones tend to last is anywhere (and the makers tend to make changes reasonably frequently so by the time one model may have a good reputation it is no longer made in the same way).
So I think the best suggestion is to go for something that fits, and try it out. I do think that cheaper options tend to last less well, but this can also happen with more expensive models.
It's also worth finding out whether there are any outlet stores locally which may have discounted stock of good quality. I'm not from your part of the UK so can't advise there.
Andrea