Do it!
Search on British powerlifting for your division - they're regions. They will have a list of comps and how to enter. You'd need to be a British powerlifting member.
See if there's a Facebook group for your division as then you can meet the lovely welcoming people.
At comp lots of people have what's called a handler, who helps you submit lift attempts, warm up, gets you to the right place at the right time etc. People will offer to do that for you if you're new and they see you don't know what's going on. They might offer in advance on the Facebook group. Don't be afraid to ask people for help. Most powerlifters I've met are passionate about it and want to help newer lifters (see how quick I've responded as evidence!)
Lifts need to be to comp standard. This means following the commands. Squatting to depth. Pausing the bar in your chest on bench. Locking out on all your lifts. Wearing an approved singlet, T-shirt and socks. There's videos on YouTube that talk you through this plus what to expect on comp day. (I can dig some stuff out when I'm not meant to be working)
If you can afford it you could get an online (or real life) powerlifting coach. They can help with all this stuff and will make a big difference but not everyone has one.
Comps in gyms are less formal but I don't know how you find them other than finding the powerlifting gyms in your area and asking around. There's no list I know of.
Comps are often live-streamed on YouTube so you can watch what they look like there. Search for your division on YouTube.
Happy to answer anything else!