A pushchair is going to be one of your most important purchase in the UK. And you'll probably own more than one before you're through.
Some things to look for:
Swivel wheels that lock-you can't maneuver around town without swivel wheels, but make sure you lock for any footpath/rough terrain type walking.
Adjustable handles-you'll kill your back if the handle isn't right. Don't buy a pushchair without pushing it first.
Weight-go for the lightest one you can-those things get heavy lugging them in and out of cars, on and off busses and up and down stairs.
The baby must be able to lie flat at first-look for one 'suitable from birth'.
Ease of setting up, putting down-does it take an engineering degree? Six hands? Could you do it with a baby on your hip if you had to?
Front facing/Rear facing-when babies are teeny tiny it's good to be able to see them and them see you. When they're a little older they like to look at the world.
Colors and washability. Go for one with stuff that comes off and goes in the washer, if there's a removable pad that you can buy extra, get it. Dark colors hide the dirt better.
Maneuverability. Go to the shop and try it out. It should go in a straight line without too much pressure, but should be able to get through small spaces and around old ladies. Not only will huge prams be hard to push, but they won't fit in a lot of lifts.
Shopping. You'll need one with a good sized shopping basket at the bottom that you can get things in easily. Try fitting a pack of diapers in.
Sun hood/Rain Canopy. If you do any sort of walking you'll need the rain canopy, and one with a sun canapopy that comes right down is a must.
Personally, I'd go with the best one I could afford. It's a good investment.