It can be a good idea to cross reference mileage and age. If you can find a car that’s consistently done 5-15k miles a year for each year it’s been on the road that’s clearly a good sign. there will be a record of the mileage progression on the MOT certificates
Auto Trader is pretty much the central resource you need. mixture of salesmen, independent sellers and individuals. some salesmen can be a bit slimy; i had a very positive time buying a car off a guy who just bought and sold cars as a side-income
don’t bother with that insurance box, definitely a money making scheme, just line up a few options and then work out which has cheapest insurance. i had an old ford fiesta to begin with and reliable cars like that have fairly reasonable insurance. mentioned above, seat ibiza is another good one for cheap insurance, good reliability, cheap-ish to buy