MrWC has a Road Angel Adventurer 7000. It's got all the usual sat nav functions for the whole of UK, and on top it comes pre-loaded with OS maps for the national parks (& you can buy the rest of the UK maps to add if you want). It's got a cycling mode on the sat nav function, plus you can get a cycle mount for it and it's waterproof.
Haven't used the OS mapping bit yet, but the thing itself has proved valuable in driving to places new and not worrying about getting lost. The thing with sat navs is to not slavishly adhere to what it's telling you - if you're in an area you know, you may well know the 'shortcut' the sat nav doesn't. But if you're in a new area and not wanting to faff about with paper maps when you're driving (such faffing with paper maps is not good if you're driving alone...) then they can be very useful.