No, not all seats will fit. The very vast majority will though, so I wouldn't worry about that. However, I would be far more concerned about comfort than anything else: Not all seats are equally comfortable, and price has very little to do with this. The good thing is you're already considering this
In terms of places to go, I'm afraid you're going to have to travel. The two best places are Demon Tweeks in Wrexham or GSM Performance in Nottingham. Both are great, DT has a slightly bigger selection but they're all fixed down, GSM not quite as many but have some rarer models on display and you can move them around in-store so you can compare side by side.
Either one will be fine, you'll definitely find something there for you. It might be a bit of a trek, but considering that you spend 100% of your time whilst driving sitting down you soon realise that a six hour drive is nothing in the grand scheme of things