Just get it done. How much money do you think you'll save? A new set of rear pads will probably only set you back £10, it's just not worth scrimping over. For what it's worth the parking pawl in the autobox isn't designed to replace the handbrake, and using it as such (particularly on an incline) seriously risks wear to the pawl and/or the transmission housing, which will cost three or four figures to repair. From your post it sounds like you know that relying on a speed bump instead of a brake is a dumb idea, which it certainly is.
Or to put it more simply: are you prepared to risk hundreds of pounds worth of damage, whilst breaking the law and driving an unroadworthy car, to possibly save a few quid?
(Remember, you'll only actually save any money if you keep the car until the pads need changing again, and rear pads in particular wear slowly. Unless you're driving like a tit you'd need to keep the car for years to have any hope of saving money).