I used these guys for Van Hire
www.northgatevehiclehire.co.ukBUT, they are more local to me than you. Having said that, they do deliver and pick up the vehicle from wherever you want if you can't collect it and drop it off yourself. I had a Transit from Friday am to Monday am for the Mountain Mayhem ride and it was £90, plus the Diesel I used. There's also a deposit that comes off a credit card, which is refunded straight back to the card when you return the vehicle (undamaged). Basically the vehicles come with a full tank and you drop them back off with a full tank - much easier than trying to figure out "at least a quarter of a tank" like some other rental companies. This means you only pay for the fuel that you actually use. Nearest one to you is Waltham Cross though, so it may be a bit far.