Satmap has 1:25000 OS as well as 1:50000. The 1:25000 maps are quite expensive (but that’s true of any GPS, as the cost is mainly determined by OS licensing fees).
I’ve had one for 4 years, and love it – more than enough detail for following rights of way, if that’s your thing (and it’s often mine). It’s the same detail as the paper OS maps.
Having said that, I live on the Hants/Surrey border, so not exactly wilderness country (although I’ve used it happily in the Brecon Beacons). If you’re in mountains or moorland, where rights of way don’t necessarily correspond to discernible trails on the ground, a bit of navigational craft is a good idea whether you’ve got a GPS or paper maps.