In simple terms, NEVER worry about a used car being 'snapped up' - there are millions for sale, you can always go elsewhere.
You're going to play an inflated price by buying from a dealer anyway, so if you like the car, research the correct price (use http://www.parkers.co.uk/cars/prices and look at similar cars for sale on http://www.autotrader.co.uk to see what others are selling for) and stick to your guns: If the dealer won't accept the price you're willing to pay, walk away & go elsewhere. Be a devil & buy at auction - you'll save a lot & still end up with the same car. If it scares you, get the car checked by a mechanic at the auction premises before bidding. Good luck.
You shouldn't pay over £5,000 for an 05 reg 3 door with 30,000 miles, and factory fitted options may make the car more desirable, but won't add to it's value.