I suspect you were thinking of something more like this
Clarke - Heavy Duty Jump Start 4000 - Battery Chargers & Engine Starters - Machine Mart
I've used them (not the one shown here) and they're brilliant, when they work. I've used the cheapys, about £25 and they haven't lasted. I suspect you have to get a better one, probably paying around £100.
If you have access to mains,I have used the "RAC" starter/charger from Argus. Stared a V8 Classic from totally flat (no warning lights) BUT the instructions say use for 10 seconds then leave for several minutes, so probably only work if a flat battery is your only problem.