Car hire is almost always too expensive to be worth it - the information below is quite old!
Sixt http://www.sixt.co.uk/ have a hire place in Olympia. Not open on Sundays.
In Sept 2008, booking for a sensible caving weekend 5 weeks in advance with the cheapest pos was Ł95.
Avis http://www.avis.co.uk have one in Earls Court, just off Warwick Way (20 Seagrave Rd, SW6 1RX), near the big Tescos. They have out of hours return.
Price ~Ł110 for same as above, Sept 08. Ł224.26 for 10day hire over Christmas for economy 4 seater.
Alamo https://www.alamo.co.uk have an office in Kennington (SW9 OJJ), and will do a micro car (website has amazing graphics, with little, med & big passengers - inviting a squeezing comp!) for Ł63. In Fulham (SW6 2JR) the same options are Ł77. Ł14 for going South of the River - hah! Vauxhall Corsa 10 day hire over christmas (winter tour) ~Ł250.
Young driver surcharge of Ł9 pd (23-24), or Ł15 pd (21-22). Weird rule where if you book for >5days, they fill up for you & charge a complete tank of petrol. Mmm, driving home on fumes to fight the fascists.
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