Flow rate and power are the two things to look for initially, length of hose is probably the next one - so many come with crappy hoses where you have to move the washer around just to do the car. Also consider the noise output, some of the cheaper ones do your head in after a while
I noticed Halfords have one of their own brand ones on offer at the moment - the chaps on detailing world seem to think they are OK, £40 off I think, only £49.99 - others:
http://www.halfords.com/webapp/wcs/stores/servlet/CategoryDisplay?storeId=10001&mpe_id=13075&jspStoreDir=HalfordsConsumerDirect&identifier=Pressure+Washers+-+c955a71f&intv_id=63035&evtype=CpgnClick&langId=-1&catalogId=10151&ddkey=ClickInfo
They also have a few others on offer, the Karcher K499M seems to be at a decent price but compare with other stores as they might just have reduced it to the same as everyone else like they usually do with their sale items They have a lot more in store but don't seem to want to tell us about them online for some reason.
I have a cheap Challenge one from Argos at the moment, it does the job, foams with a gun, has decent flow and pressure but the hose is a couple of metres too short, it's very lightweight and a little noisy - but it functions perfectly OK.
Depending on budget there are lots to look at - if you are going to give it some hammer, use it a fair bit and want it to last it might be wise to look at a decent Karcher (someone on here sells them but seems reluctant to do us a group buy ) or a Kranzle perhaps?
I'd like the Kranzle K1150T or Karcher K7.85M - the hose reels are something I want on my next washer because I'm fed up with trying to store my others without them springing all over the place