You dont mention what time of year you're are looking at but for a cheaper deal the main thing is obviously to avoid the main summer holidays - If it's just two of you I assume school holidays are not critical.
Otherwise just keep scanning the various website (the most complete listing is at www.ukmotorhomes.net) for an off-peak special offer.
All the rental firms find it easy enough to rent out campervans in summer but are always keen to get them on the road for the rest of the year so you should be able to get a much better deal than �600 outside of peak season.
As an example, at the moment you can get a new VW Go-pod campervan from �50 / day in Scotland if you are able to get away before Easter. Or up to the end of April you'll get them for �69/day = �483 per week.
Two last thoughts on prices of campervans -
a) do consider the fuel economy of the van you choose, especially if you are planning to drive a long way.
b) they will inevitably not be as fuel efficient as your car so it may be worth picking up a van in the area you want to tour - so if it's Cornwall you'd probably be best to drive down in your own car and pick up a van there (plenty of classic looking VWs for hire - all of which have much more living space than a Bongo, and tend to cost a bit less).
Happy hunting :-)