we hired a v cheap van to get the cider apples last year. Blake/Bloke/Biro did it, and I'm sure it was local (?TBR area?). Maybe you can pm them and find out the details. I just remember it was a really good deal (and that we got a diesel van which meant that fuel to The Forest of Dean and back was cheap too). Sorry I can't help more.
I used thrifty a couple of times and carpoint once when I lived in SE1, both were fine.
However never ever EVER use Astor Van Hire (they're not in SE1, they're in Bow). I once used them when desperate because another company I booked with had their vehicle returned crashed, and Astor were the only company that had something available at short notice. Much grief ensued with this disastrous hire -- including being stopped and arrested for driving without a current tax disc, the handbrake not working, and the whole heap breaking down and not being covered by the AA as they had claimed... and me having severe humour failure.
So always check the tax disc on any vehicle you hire as well as any damage that's already there. I never even thought of this until I found out the hard way!
And another thing with vehicle hire -- I would recommend booking on credit card because if the cost is over 100ukp and it all goes to pot as with me and Astor, the credit card provider has an amount of joint responsibility with the hire company (this way I eventually got my money back with compensation and may have been protected for any fine from the DVLA -- they let me off this time but technically you are responsible for checking the tax disc!)