As for age and mileage, if you had to choose, I'd assume it's better to have less mileage than less years?
Personally, I'd go for the other way round: if it's not done many miles over a long period of time, that either means it's done very short journeys (not good for the engine) or it's done longer journeys then been stood around for a while doing nothing (some bits still decay while doing nothing, or can throw up problems when it starts to get used more again).
As elvis said, if it's got a good service history then it's a good sign it's been looked after. If you find a car with, say, 12000 miles on it, it may never have had a service - that won't have done it much good, even with that low mileage.
AA Vehicle Inspections - I put my registration in and it came out at £291, but it's a bigger engine and possibly more complex than a Corolla or Yaris.
RAC Vehicle Inspections - this gives some guide prices, you're probably looking at £150-£200.
If you know a good local independent garage, you may be able to get someone from there to give a car a look over for less than those prices.