Stick your postcode in here: http://www.samknows.com/broadband/exchange_search
that will tell you what exactly is available to you at your exchange.
Not sure how techie you are and how much research you have done - I'm guessing quite a bit as you are looking at BT infinity. However, here's what I know, I'm guessing you have already covered it but it may be of use to others.
As I'm sure you know - Broadband speed is limited by the copper wire to you house, the closer you are the higher the attainable connection.
If you are on a normal exchange the very maximum is 8000kbps (or thereabouts) (8Mbps)
BT started upgrading their exchanges to 21Cn recently which gives a top connection speed of 20000kbps (20Mbps)
For a more immediate increase in speed - that costs nothing - try plugging your router into your master phone socket.
You can also disconnect the bell wire (that made the old analogue dial phones ring)
info is here :- http://www.jarviser.co.uk/jarviser/bell ... shell.html
an alternative is to fit a BT iPlate - more info is on the same site.
and if its possible remove anything else that's connected to your phone line anywhere in the house. (Even just to test)
Finally, and a little bit more technical. if you have a Netgear DG834 router (which is one of the most common ones) then you can get custom firmware to get the most out of it here :-http://dgteam.ilbello.com/index.php?pid=12
with this you can set the SNR (signal to noise ratio) lower meaning you can get a higher attainable connection (ie nearer the maximum your line can give) but the trade off is a slightly less stable connection - default is 100%.
Mine is set to 5% and I haven't dropped in over 170 hours and am getting 7616 kbps - but in my case the exchange is very close.