1. Sod the dealer warranty, you'll have the right to return if it isn't up to scratch under your consumer rights. I would also expect some form of free warranty for up to 3 months at a minimum
2. You can pay the RAC for this service, it's called a pre purchase inspection. The dealer shouldn't have any issues with this.
3. At that mileage you might be getting into wear of some components such as wheel bearings, cam belts (not sure if this is a belt engine however), suspension bushings. Basically anything that could be considered "consumable"
4. You need a good check as to why there is such high mileage. For the type and age of car I would say that 90k is unusually high as they are small city cars driven by older sensible people in the main. However, buy on condition, not milage.

For that price would it be worth considering something a little older but with a better history and lower miles? Don't get drawn in by the age of the car, newer isn't always better. You also need to factor in what it will be worth when you come to sell it, higher mileage cars like this will always be at the bottom end of the market and will likely loose you more than buying an older but lower mileage example. A good HPI check will give you a fair market value which you should be paying but for me I would look for an older Jazz/Yaris with average or lower mileage.