Whats the limitation of newer units? Garmin seem to do one with an electronic compass, higher definition screen and a camera (which seems a bit plain silly to me). Can you view and browse 1:25 maps in OS style? Can you set multiple markers for areas of interest? What is the battery life like?
I got the Garmin Oregon 550t with 1:50 000 topo maps of southern Sweden. You can buy 1:20 000 maps as well. I don't know what is available in UK. It has a 3d compass, so it is reliable even if not held absolutely level. Also a camera, very nice to take geotagged pix that you can browse using google earth. Every picture you take becomes a marker for that spot, so you can actually navigate using the pictures for reference. If you see something nice or interesting, take a picture. Later you just browse to that picture and can view where it was taken. Another key press and you start navigating to that spot. You can mark spots without taking pics, but that is no fun... The pics are OK in good conditions (daylight outdoors) but not by a far way as good as with a "real" camera. Still great fun!
The GPS is really nice for geocaching and on a bike. The display is bit hard to read in the sun, even if you crank up the light level.
Edit: Just got one of my pics i took yesterday selected by google maps.

Had one selected previously, but that was manually geotagged. Here are some sample pictures taken with the GPS: (Click to see full resolution unedited original pic.)
The specs for the GPS say 16 hours run time. But if you crank up the light and increase the time before powersave starts and use the camera now and then, it is more like 8 hours normal use. Two rechargable AA-batteries, so no problem carrying extras. If you turn it off and only take a few positions a day, or in emergencies, it should last for weeks.
I bought a holder for the handlebar on my bike. Best bike computer I've ever had. (And most expensive...)
But I still carry a normal map and compass. Much better for a better overview and long distance navigation.
I also considered a camera with a built in gps...