Just be a little careful about relying on the results from these online checkers, as they're rough guides only and not to be relied upon as a definite answer. You'll also find that some of them contradict each other, depending upon how much information has been fed into their databases.
For anyone who already has a BT line, the BT website has a checker which has the advantage of being pre-programmed with line characteristics for many areas. Even so, it's still not to be taken as 100% certain:
http://www.bt.com/broadband/bb_info.jspThe speed of your connection is mostly down to the capabilities of the Local Exchange in your area.
In densely populated urban areas where nobody is far from a central office, that's true to a large degree, but it's not necessarily so in more rural areas, or even in towns which are fairly spread out. The line between the C.O. and your house can have a big effect. For example, on my local exchange -- Stalham -- folks right in town can get speeds up to 2M or so, but out here at the far end of 5 miles of wire we're down to about 0.5M.
In some areas there is also the problem of pair-gain units (BT calls them DACS). Without getting too technical, where demand for phones has outstripped the installed capacity of the cables, the phone company has used a technique which allows more than one phone line to operate over a single physical pair of wires. (It's a much cheaper short-term solution than running new cables for miles.)
While this system works fine for normal voice communications, broadband won't operate over a DACS/pair-gain unit; you must be on your own dedicated pair back to the central office.
So even though the lines themselves might support DSL, there might be logistical problems if lines are in short supply. Sometimes BT (or whoever) can transfer somebody else without DSL onto a DACS line to free up a pair of wires for your new broadband connection, but if there are no spare pairs available to be able to assign a a dedicated line to your house, there might be a wait.