Yup, if your current package allows you to upgrade at that price then go for it!
bare in mind, though, that if you're already on a bundle deal it could cost as much as an additional £7 to upgrade to the top L tier.
For example. If you have 3for30
BBI:medium
Telephone:medium
tv:large.
Then effectively you're paying £9.50 for your TV and £9.50 your Broadband, then £11 for the line rental. So by upgrading the BB to L (for as much as £7 - could be less) you'll still only be paying a maximum of £16.50 for the broadband.
A little confusing but i just wanted people to be aware that they won't necessarily be able to ug fom M to L for £2.00.