I got the legend hcx post free for 130 from amazon a week ago.
roger's deal with the uk map seems the best there is to be had if you're sure you only need the uk map, at least from what i could see from searching and searching for deals. One thing that put me off it was that it was an sd card and I think it's best to have a dvd map so you can use it to plot routes in mapsource on the pc, which is Garmin's proprietary software that comes with the gps unit.
as for other maps, you can use openstreetmap's free open source maps. They don't have all the roads and you can't use it for on-road route planning, but if you upload routes made, for instance, on google maps, to the gps that won't be a problem.
(of course, some criminal types simply download an unlocked garmin map via bittorrent for absolutely nothing, but that is illegal )