As long as no one mentions that they have virgin media fibre optic we'll be alright* :|

If it's the cable service and it's seriously shite, then give tech support and get an engineer out to have a look at it. But its worth doing a number of speed tests at different times of the day, over a period of a few days, as that'll tell you if it's a constant issue or whether it's happening during peak times. If it is the latter then it's most likely a over scribed UBR and there isn't a lot that can be done.
It could also be an issue with your cable modem or a fault down at the 'green cab', in which case i would have a look on the cableforum.co.uk forums and look into reading your downstream/upstream levels (hot spells can cause issues with fluctuating levels which cause drop outs, lag etc).

If it's the ADSL service then tech support!

*It's not FTTH, the last 100m+ is coax. It's FTTC!

There is nothing you can do.

How slow is slow? Are on the cable service or the phone line one?

Currently I am with Virgin media and the internet speed is slow, so I was wondering was there any methods that I could use to increase my internet speed? Thanks.