Take a trip down to Sainsbury's you may be able to get a Sony NAV-50U for £99.99 It's an discontinued model reduced from about £249. It lacks some of the features of newer models but at £99 you can't grumble. This also has a much better screen than the Garmin i3.
If you start to ask questions about features etc you'll soon end up spending £250
EDIT: Teach me to read! this does not do full postc0de search but it does have the facility for partial post code search. You'll also be very lucky to get speed camera database as standard for this money.