Sorry PQ you're putting 2 and 2 together and getting 5. Hire car depots in non-tourist areas such as Guildford and Reading are there overwhelmingly to service the needs of people like me - businessmen who hire cars to go on business trips, NOT the very occasional American tourist trying to avoid a London return. The depots are normally located on industrial estates on the edge of town and are invariably weekday 8am-6pm operations. Sure they will be able to arrange in advance a pick-up/drop-off at another location or outside these hours (eg Sunday), but you will have to pay an additional fee for this. All of this can be simply avoided by returning to LHR or using one of the few parkway stations suggested by Flanner and myself.

Another Question: Is 5th Gear reputable for car hire ? I'm not familiar with them and am not used to providing a deposit.

Never heard of 5th Gear. Googling them produces several outfits with similar URLs. These people are all brokers who don't hire the cars themselves, they just act as intermediaries for the hire car companies (Europcar, Avis, Sixt etc). If you want to use a broker, it might be better to go with one of the better known operations such as AutoEurope or Holiday Autos.

Well if you cannot possibly drive in any British city why rent a car? Take the train instead i guess.