If you've got wooden doors fit 5 lever mortise locks. If you're replacing euro-locks choose a size that is slightly longer than the length from the centre of the securing screw to the outside edge of the handle - you don't want the lock sticking out more than 5mm or it will make it easier for a thief to snap the lock off.
If / when I get around to replacing my euro locks, (which as they came with the doors, I doubt even have anti drill or pick features) I'd want ones that are certified drill, snap, bump, pick and pull resistant, otherwise it would seem a waste of cash.
I had a google to see if there were some more affordable locks than last time I looked -
UAP are offering a "free" sachet of Pickbuster plus about £5 delivery so you can treat your existing cylinder locks to make them resistant to bumping.
http://www.universal-imports.com/view-details.asp?scat=0&page=&keywords=&ID=1087
The site I link to earlier has a very cheap anti-snap lock
http://www.handlestore.com/keycylinder.php @ £12 + delivery & also includes a free sachet of "pickbuster" solution. Although I'm a bit dubious about such cheap locks and about relying on an oil to dampen the pins.
I noticed screwfix now have some anti-snap locks for about 20 quid, but they're not anti-bump
The KABA GeGE bump-proof locks looks much more promising as its not hugely expensive - still around £50 though. It is kite marked - which may be important for insurance purposes in the future and has actual anti-bump pins
http://www.ironmongerydirect.co.uk/...eGE_Anti_Bump_Anti_Pick_Anti_Snap_Cylinders/# but after looking at the manufacturer's site, I think the lock itself isn't snap/pull resistant

, it appears that you need to fit a security handle which shrouds the end of the lock cylinder to prevent it being snapped off or pulled.