Hi,
Closest I can think of is
www.lixxus.co.uk
They have a Start Basic PAYG service that doesn't charge if you don't use it, the downside is that according to the FUP, you can only have 2 months (60 days) of no-usage periods in the year.
Alternately,
Now that BT have a startup and cessation fee on broadband, it's almost worthwhile finding a provider that charges around 10-12 PCM or one that gives a discount if you pay up-front for a years service (Namesco, Newnet both have a "lite" package) the total for a year might be less than the cost of 4 months service plus the startup and cessation fee!