As per above avoid Car Supermarket type places like Carbase etc, you might get a good car/service but chances are you won't.
Again main dealers can be hit or miss, if you're a cash buyer they are less interested in you as they want you on finance.
In my experience the best garages are privately run independent garages who care about your custom and know the cars they are selling.
For your budget and requirements above a good choice would be a 3/4 year old Ford Focus. For that money you will get a decent spec car with low to average mileage and you'll have a choice of hatchback or estate.