There have been endless discussions in the media about "up to 8Mb" packages over the last 2-3 years! It has been to the Advertising Standards Authority on several occasions, but the ISPs are still allowed to get away with it.
As to your actual issue - standard ADSL packages are restricted to a max.upload sync speed of 448kb/s - this would give a max. connection speed of ~ 384kb/sec.On a d/l sync of 2.5Mb/sec your line will be profiled at 2Mb/sec - you will actually be able to get about 85% of that which gives you 1.7Mb/sec.
If you desperately need a higher upload then you need to look for LLU suppliers who will allow a higher upload sync. Are you out of contract with BT?? If so, find out who (other than BT) has a presence at your local exchange by using this site.
http://www.samknows.com/broadband/broadband_availability