Possible levels of GPS
1) Non-mapping
Non-mapping GPS units are a lot cheaper, but they are limited to guiding you along routes that you have set up on the PC at home and downloaded onto them. This is done by following an on-screen arrow. Routes are set up either using a mapping package such as MemoryMap or Tracklogs (about £100), or on a website such as bikely or toporoute (£free).
If you get off the prepared route, then it's either backtrack where you've been (guided), or navigate by guesswork and the knowledge that the route or destination is XX miles thataway.
A GPS of this type is likely to be about £100
selection:
http://www.gpsw.co.uk/cats/cl3_23.html
Basic GPS only
http://www.gpsw.co.uk/details/prod3426.html
same plus PC cables etc
http://www.gpsw.co.uk/details/prod3424.html
2) basic Mapping
Basic mapping GPS units navigate as above, but also show an on-screen road map of the surrounding area with a "you are here" pointer. That means that if you get off route, you can see which roads to take to get back on route.
However it is necessary to get the mapping data to load onto them separately.
Suggested offer
old model, including mapping data (£100). Note that the included PC cable is RS232 serial, so if you have a recent PC you will also have to buy a USB adapter or get the offer with the included USB converter cable. Compared with newer models it doesn't have a lot of memory, so the area of mapping that can be put on it at one time is limited.
http://www.gpsw.co.uk/details/prod3429.html
3) Mapping with automatic routing
These can work as above, or in the same way as an in-car Satnav, directing you to your next (intermediate or final) destination along roads of its choosing, using on-screen prompts like "in 200m turn left into Church Lane". If you get off route it will work out a new best route and direct you on that instead. Longer routes are set up using several intermediate destinations in places chosen so that the shortest route between them doesn't direct you along major A-roads.
The downside is that they are generally more expensive, and that a more expensive version of the mapping data is required (£140)
Suggestions
a)
http://www.gpsw.co.uk/details/prod3166.html £160. Doesn't include any PC interface cables or what have you. It's suggested that you get the mapping data on memory cards at £??? a go.
b)
http://www.gpsw.co.uk/details/prod3038.html £200 Includes everything except the mapping data (even a bike mount).
Mapping data (for whole of Europe)
a) basic
http://www.gpsw.co.uk/details/prod580.html £100
Normally, this does do autorouting on the PC, but not on the GPS. I have however heard of fiddles to download the required info onto the GPS, so you get the same results as (b) but without the extra cost. Google is your friend.
b) autorouting
http://www.gpsw.co.uk/details/prod2340.html £165 includes extra mounts & power cable for use as car satnav, which seem to always be bundled in.
c) autorouting on card (UK & Ireland) £60
http://www.gpsw.co.uk/details/prod3068.html
(it says it's for the streetpilot satnav, so I'd want to check it would work in one of the above)
d) MemoryMap/Tracklogs is OS mapping data on screen. It's much nicer to plan routes along on the PC than (a) or (b) are
If you don't want turn-by-turn guidance and are willing to follow an arrow on predefined routes, the offer linked under (2) looks good.
If you are willing to buy from America via eBay, there are also regular good offers like
this
Caveat:
I use one of these
http://www.gpsw.co.uk/details/prod3041.html with the autorouting data linked above. I have no direct experience with any of the above GPS units.