If its an official main dealer then they will give you 1 year cover anyway. You can extend to 2 sometimes and they will give a better price for 2 years than if you want to put a warranty on it next year after x amount of milage/1 year older car. I know Merc/Audi sometimes have 2 year warranty offers at certain times of the year so worth checking.
If its a non official dealer then the warranty quality really varies. Warranties like RAC, AA etc. all have big clauses in them such as large excesses, maximum coverage amount, maximum claims per year etc. So check the details especially if you are buying an older/expensive car.
The best non official warranty i found was with Allianz (and they are the underbroker for the official warranty of my car) and you can get a zero excess option with v large maximum claim limted for a decent price.