The key word is 'upto', but it doesn't guarantee that you will get that, because it all depends on your line, your equipment and the exchange plus the ISP's contention ratios. Ofcom are now making it a rule, that ISP's have got to provide more correct information on the true speeds that their customers should be able to receive.
I use TalkTalk, and was getting speeds of about 1 to 1.5Mb, but with BT's and TalkTalk's help in changing various parts,I am managing a little over 5Mb now. TalkTalk speedtest states that I should receive 6.5Mb from the exchange, but a direct test by BT states maximum of 6Mb, and I am one and a half cable miles from the exchange.
Try a number of different speedtests, including the one in the link (which TT technical suggest)and see if they are compatible with each other. You could also do a ping test, mtu test and trace test to see if your are having loss breaks on the lines. Do this at various times of the day, because it can fluctuate, depending on traffic.
Broadband speeds are a complicated issue, and you can find out more from http://www.kitz.co.uk/sites/site_index.htm
When you have all the information to hand then take the matter up with Talk Talk, and if need be, go to higher level complaints. Your profile would need to be set and adjusted remotely by TalkTalk, if its wrong. To see your profile, check what router you are using then use the code and password, which should open a page with all the information on your router and service connection.