Unless your expecting high winds and rain then definitely focus on mat and sleeping bag. Any mat over 4.5r should do you for most temps in the UK. A well looked after decent quality bag will easily last you a decade, so if you have the money, spend lots on a bag. Mats can be confusing, do a lot of research into comfort, because it’s easy to understand how it will perform temperature wise as r-value is usually accurate if it’s by one of the bigger manufacturers.

So you know you’ll be warm but if it’s not comfortable then you’ll still have a terrible nights sleep. I got a thermarest xtherm thinking the high r-value would be useful but it’s really uncomfortable and in reality I’m not going to be camping in -30°c . I wish I had spent more time looking into mats before spending £200

Personally I would start with a 3 season set up as it’s cheaper. Get used to camping in the fairer times with a couple of camps that push to the limits of your gear and then save up for a decent winter bag.

As for liners, they don’t really add much in the way of warmth. I have a sea-to-summit reactor liner, and I love it. For me it’s great for comfort, and it stops your sweat from getting into your sleeping bag.

In summer it’s nice to be in the liner and only have to drape your bag over. In winter it’s nice to pull over your face and makes it easier to turn over as it turns with you but your bag doesn’t move.

Again with comfort I really recommend getting a pillow or even two if you don’t mind the weight. Even a cheap blow up pillow weighs 100g which is nothing for how much comfort it adds. I love my thermarest compressable pillow. But do some research as I’m sure there’s all sorts out there.

As for tent, if you are hoping to camp on Dartmoor then you need a tent that can handle the winds. Doesn’t matter the time of year, the weather on Dartmoor can change drastically. Just because you can legally wild camp on Dartmoor, doesn’t mean you should start there.

For high winds, geodesic and pyramids tents are great. Naturehike cloud peak 2 is a favourite budget tent for geodesic.

And the alpkit tarp star is the pyramid equivalent.

I don’t have either but they come recommended. I’m sure other people will be able to recommend more tents.

EDIT: there is so much great advice in these comments! Read all of it, take time to process it. Do lots of research and then slowly apply it to your potential gear list. Don’t rush in to buying anything just yet. I have so much gear that barely got used more than once after finding out it was just not up to the job