There are legal issues surrounding mountain biking in the UK. I’m sure there are legal issues surrounding riding elsewhere too, but I’m a fair bit less familiar with them, so I’ll limit my discussion of them to saying: make sure you know what they are before you ride.
The bike, and you.
There are certain legal requirements that you and your bike should meet for riding on the road. Your brakes need to work. If you’re riding on the road at night, you need to have a white light in front, a red light to the rear, and a red reflector at the back. Orange pedal reflectors are also a legal requirement, although the law is silent on how one attaches these to clipless pedals. You can’t ride on pavements, you can’t run red lights, and you can’t ride in a manner that is unsafe – which includes under the influence of drink or other drugs.
There are also a whole pile of things that the Highway Code specifies that you should do (but aren’t actual legal requirements), including wearing a helmet, wearing reflective clothing, keeping your bike in good working order, and not carrying anything that would mess with your balance.
To be honest, before you ride on the road, it’s worth checking out what the Highway Code has to say. Whether or not you stick to the letter of the law, at least then you’ll know what it is.
Of course, some manufacturers get around the legal stuff about selling bikes (they must have reflectors everywhere and a bell) by attaching a conspicuous sticker to the bike before it’s sold that informs everyone that the bike is ‘for event use only’ and therefore not subject to the same restrictions.
Essentially, these will be more or less exempt from most of the legal issues around riding. So long as you stick to the waymarked routes, you don’t need lights, you don’t need a bell, you don’t even need to worry about rights of way and with any luck you don’t even need to worry about pedestrians. Good sense should still rule though, so wear a helmet and appropriate clothing, and keep your bike in good working order.
Equally, some trail centres are governed by bylaws or have their own rules. Bike Park Wales in particular makes theirs very explicit – you must wear a helmet and so on. In other places it’s less clear, and less obviously policed, but nonetheless be prepared to face some difficulties if you’re stupid enough to ride without a helmet, or with a knackered bike.
Here, once again, you need to pay attention to the law. I’m not saying for a second that everyone always sticks to the perfect letter of the law, but again it’s worth knowing. Equally, in our efforts to make sure that cyclists and mountain bikers are respected as road and trail users, it’s worth being seen to ride considerately and legally.
In England and Wales (and I think also Northern Ireland, but I’d be inclined to check first…) it is not legal to ride on a footpath – these are for pedestrians only. It is, however, legal to ride on bridleways, restricted byways, green lanes and roads. These are what make up every natural route you’re likely to ever find directions for. Bridleways seem to often be broad, stony, and relatively uninteresting tracks, and this is one of the reasons why using routes that other people have planned out is an excellent idea – they’ve done the work of exploring the area and weeding out the tedious or unrideable bridleways, and put together a route of more interesting and better trails.
Incidentally, the rights of way for cyclists don’t change on Access Land. Stick to bridleways, and quietly envy the walkers who can head out into the trackless wilds.
In Scotland, however, the Right to Roam laws apply to cyclists as well, so it is permissable to ride on paths and tracks pretty much wherever.
Wherever you decide to ride, make sure you do it considerately and safely. Give way to walkers and horses. Close gates. Don’t ride over planted crops or interfere with land management in any way. Don’t try to ride through logging work. Have some sense.