Where can I ride...?
Off road cyclists' can legally cycle on the network of byways and bridleways across England and Wales. For families and beginners there are also a number of old railway tracks and canal tow paths across the UK that often form part of the national cycle network  and make for excellent training ground.
Reservoirs, forestry commission land and mountain bike trail centres also offer a range of trails with something suitable for all abilities. The mountain bike trail centres across the UK colour code trails (similar to ski runs) to give you an idea of how technically challenging the trail is and what standard your bike will need to be to ride them.
Though there is not a national standard, most trail centres use Green/Blue/Red/Black grading system where green is the easiest and black the most technically challenging which should only be undertaken by experienced off-road riders.
Public Rights of Way open to off-road cyclists:
Cyclists', Horses/riders and Walkers all have access to Bridleways. Cyclists must give way to Horses/riders and Walkers ensuring to be considerate to all other users. Horse drawn vehicles are not allowed. Motorcycling is not allowed. Bridleways are generally unsurfaced, and due to this some can become almost impassable for Cyclists, this generally applies more to the winter months or after a lot of heavy rain.
Byway open to all traffic (BOAT):
Cyclists', and all wheeled vehicles are allowed, but generally a BOAT is used by Cyclists, Horses/riders and Walkers. Vehicles such as 4x4s also use BOATs, and in some areas you will find fairly high usage. Where this applies BOATs can often be found with deep groves in them making them heavy going even for the fittest of cyclists, as with Bridleways the winter/wetter times can often present difficult riding conditions.
Restricted Byways first appeared in the Countryside Rights of Way Act 2000 (CROW). They allow a right of way for Cyclists', Horses/Riders, Walkers and for any other non-mechanically propelled vehicles on former Roads Used as Public Paths (RUPP).
No legal definition is applicable with the phrase 'Green Lane'. The term generally refers to unsurfaced/unclassified country roads. These are depicted as 'white roads' on Ordnance Survey Maps (OS). However, some 'white roads' are private and you will have no legal right to ride on them.
Permissive paths give landowners the power to redirect the path or close the path. Permissive paths must be signed if they are not to be used. Landowners may dedicate a Permissive path to cyclists only, Horses/riders only, or maybe all three groups where they so wish to do so. Permissive paths are often found on Local Authority/Railway authority or National Trust land.
Please see the 'Scottish Access Code'  for access details to Scotland's countryside.