The simple answer to the question can you ride your mountain bike in the rain, is yes. Whether you want to or not is another question.
You may have heard the maxim: "there's no such thing as bad weather, just the wrong clothes. It's annoying in many ways, and particularly because it's true. Heavy rain, strong wind and no anorak is not only very uncomfortable, but can also be dangerous. None of us want to end up with hypothermia. But if you're carrying a decent waterproof and the right number of layers to keep warm, then it can be a blast.
For example, I still have great memories of the first day of a holiday in Spain when we were caught in a serious downpour. The ride from Sentenil back to our base in Monte Corto - normally a gentle introductory ride - was made more challenging as a stream that was generally a trickle, rose and flowed faster making the numerous crossings much more interesting, and for me anyway, more fun.
So what are the right clothes?
Of course it depends on where you are riding, what time of year it is and how far off the beaten track you're going, but here are a few basics:
1. A waterproof jacket
If you're an all weather rider, and especially if you plan to go to remote places and mountains it's worth investing in a decent waterproof that is breathable and keeps the water out. I have a heavier Goretex jacket for the big rides and bad weather, and a lightweight jacket that's water resistant and packs down small for shorter rides and dodgy summer days.
2. Waterproof shorts
These are a great development. They are perfect for keeping your bum dry on muddy rides, and when you're splashing through puddles, and are also good in the rain, as long as it's not too cold, which is when you need...
3. Waterproof trousers
Now I've got waterproof shorts I rarely wear my waterproof trousers, but they are occasionally dragged out of the cupboard. As with the jacket, you want them to be breathable, otherwise you'll be dripping sweat. Also they need to narrow at the bottom, so they don't get caught in the chain.
Rather than one thick top, it's best to have several layers of lighter ones. This makes things much more versatile. Being zipped into waterproofs can be too warm, as well as too cold, so it's always good to have options.
5. Socks and gloves
I don't know about you, but for me it's the extremities that always feel the cold first. In fact it's my feet, whereas my husband always gets cold hands. Waterproof socks definitely make a difference, but they aren't foolproof. Despite a silicon seal around the top, water does sometimes get in if you're doing river crossings, or splashing through too many puddles. Once in, it sloshes around unpleasantly, unable to get out again! Despite this, if you have 5/10 shoes, waterproof socks are a must have, because the shoes soak up the water in a big way. Back in the 90s, when I started mountain biking, we lined our shoes with plastic bags from the supermarket, and that was pretty effective too. It also gives a single-use plastic bag a second life! If you're prepared to spend a bit more there are lots of good winter boots for spd pedals, rather less so for flats as far as I can see.
If it's your hands that get cold there's plenty of choice of wind and waterproof gloves.
Typical British weather means that you'll be taking stuff off and putting it on again, so it's good to have a rucksack to stow it all in when the sun comes out!