I laid and maintained my own moorings for a number of years. The advice above is good, but be aware that even in areas with no significant current the bottom of the riser, where it lifts on and off the sea bed with the rise and fall of the tide can be subject to the most wear, especially on a sandy bottom. I have seen chains that look good, until you push the links apart and see the wear/sand abrasion on the faces between the links.
As long the boat's sitting in salt water, just leave it alone. You're sitting in water, so the interior of your boat will never get much colder than the the temperature of the water. Sea temperatures around the UK never get below 0. If you really want to do something, take the water inlet off the seacock, dunk it into a gallon of antifreeze and run the engine until it comes out of the exhaust, but I never bothered in Portsmouth Harbour. It's far more important to do it if you're sitting on the hard, where icy winds blow.
Just out of curiosity, where is your mooring and why is it in such a strong tidal stream? Is there no where else that would be quieter? Wind over tide must make the boat very uncomfortable.
I think rather depends where your mooring is and how exposed and secure it is.
It's too soon for me to be able to offer sage advice but here's my two penn'orth. My boat is spending its first winter on a Scottish loch swing mooring; I just got back from an overnight visit after three months (she's 300 miles away from where I live) and I found everything to be in order. My insurance covers it; and I've wedged ajar the forehatch to promote ventilation through the saloon (there was only a very little mould). The boat has a small solar panel (I'm guessing 100w) and the batteries were fully charged. I'd intended to remove the sails but conditions on my last visit of 2023 didn't allow it; on my return they were still secured as I'd left them (despite all the accounts of sails unwinding and being destroyed it really shouldn't be difficult to prevent this occuring). Again, I had intended to run some antifreeze through the raw water - but, like another correspondent, I hope she'll be okay given the relative warmth (and saltiness) of the water she's in. The cooling circuit has AF in it as standard. As someone else said, it's a risk judgement (and it involves what you feel you can afford given all the other things you want to do in life). My boat is only worth four figures; the situation is (relatively) sheltered; there's only a knot of tide; the bottom is mud; I've 40' under me; the tackle was inspected last year - all this makes the cost of annual winter parking unreasonable for me. I recognise that there may also be a less rational motive behind my decision too: I enjoy second-guessing the judgements of others. Like any 'community' there's an element of shoal behaviour - my boat's all alone on the water now. Time will tell.
Typical behaviour for moorings that use a single weight, especially if the chain is shortened by higher than predicted sea levels.
I am a Dart local too, and am about to buy my first boat (hopefully 23'), having not sailed for many years since having children. Can you give me any helpful local advice - which moorings to choose, recommended local boatyards, good insurers...? Any help appreciated.