PDA based units may/may not suit you, but they are worth a look. Some are wired solutions, with spaghetti all over the dash (unless you can bothered to try and hide everything) and others are wireless.
The newer generation of standalone units are very good value around the £400-£500 mark. Each has it's pros and cons, but I believe that all represent good value for money.
Personally, I went with the TomTom Go solution and am very happy with it for the money. Most PCWorld's have one on display if you want to take a look at one.
Read up on all of the different device as sites like
http://www.pocketgps.co.uk/ . The forums at their site are very informative too.
Most devices have sizeable Points Of Interest databases which contain everything from cash machines to petrol stations to beaches to fun parks. Navigation to your closest petrol station is about 4 clicks from the default screen in TTG. TTG is also releasing an update tomorrow which adds Custom POI support, for things like Speed Camera locations.
Postcode routing is not perfect on the TTG, but the first 3 or 4 letters/digits works ok. On the PDA based units, there are Postcode routing add ons which probably work better.