Haven't used XLN just because since I work from my home office now I use the home broadband.
If the pick was XLN vs TalkTalk though, from my TalkTalk experience I'd go with XLN.
Regarding different speeds from different suppliers, generally speaking, that's not a silly question. You have your standard broadband with all suppliers and most will run a test and guarantee you a speed bracket at the point of signup and if they can't provide that, you get out of your contract free. If they openly don't come out with a speed estimate - ask for one. Fibre broadband is your next step up and generally speaking there will be a max limit that will come out the local exchange to yourself. For example, the limit might be 50mb but the company will sell, 50mb, 100mb and 200mb plans for example. Obviously, no point signing up for a higher plan if the max you can get is 50mb. Again, you should be able to get an estimate from them on what they can provide. 30mb - 50mb is more than sufficient for most needs. All suppliers except Virgin rent their hardware from BT or use their own hardware but it's using BT cables regardless.
The only real standalone is Virgin and they're unique because they're the only supplier to offer a true end to end fibre connection straight to your premises and can give you whatever speed you actually order as they're in full control and don't use BT's cables or hardware. The downside is it comes at a cost but you don't need to pay phone line rental with their broadband whereas you do with everyone else. If I could get Virgin here I would but living in the country they haven't upgraded the local box to allow more clients
I hope I didn't bore you with my extremely geeky explanation