When exploring this option, before I went down the road of FTTPoD and willing to pay a few thousand, I tried Amvia and found they got a little persistent/pushy with me (typical sales tactics for some companies I guess, so can't really blame them for trying to get me to buy).
With leased lines there can be different providers (some of which are either on-net or off-net), there are also different speeds which you can pay for (such as 10 megabits over a 100 megabits bearer). The bearer is the maximum speed your connection can support, the link between you and the exchange/PoP pretty much.
http://linebroker.co.uk/ offered a variety of options last time I used it. I think CIX's website also does. Most of the leased line quoting websites insist on them having to have your contact details first, whereas there are a few websites out there which can give you some instant quotes without needing contact details first.