The connection speed is the factor that determines how fast you can download web pages, files etc. On the other hand, download limit, if it is mentioned in your package, you are supposed to check that the total download in a particular month does not exceed that figure.
After taking your requirements into consideration, you should select a plan that can offer you the best connection speed and download limit.
Say, you are a light user, then you would require 1 or 2 Mb download speed and you would not require usage allowance more than 5 GB. You can go for 8 Mb broadband, if you want a little more fun, say, you like to play online games using consoles.
Faster broadband (10 Mb or more) packages are a bit expensive, especially, when you combine it with unlimited usage allowance. Unless you are a heavy user and a regular downloader of huge files or have plans to setup
home networking, it is not advisable to sign up for such packages.
In my opinion, BT Total Option 1 is one of the ideal packages that combine speed (8 Mb) and nice download limit (5 GB) and also costs just £17.99 per month.
You can also compare varioius broadband deals
here.