My local branch in Tooting has these for 13 quid. Small and light SMPS type which can do 6 or 12 volts, all types of lead acid including SLA gel types, pulse charge for attempted recovery of deep discharge, and a maintenance charge so it can be left on indefinitely. Peak output is just under 4 amps, so it should also be able to start a car ok after an overnight charge. 3 year warranty.
Has fixing holes, so it might be suitable for permanent fixing in a little used car, with some form of mains inlet mounted under the bumper, etc.