Finishing paint/varnish repair

If you coat it in epoxy, at least you can still see through it and tell if moisture's got in, but if you put glass fibre on it it'll rot away underneath and get so bad before you notice that you'll end up needing to replace whole panels or scrap the entire hull. Using epoxy is more like using varnish but with it lasting longer, the main downside being more dangerous fumes and higher cost. It's also best to apply this when the wood is maximally dry, so winter isn't the ideal time for that kind of work due to all the extra damp, but if you can get it warm for 24 hours to dry it out before putting the first layer of epoxy/varnish on you should get a better result. Applying the warmth with ordinary heaters may not work well as heat rises instead of going down where you need it, so it may be worth trying a £20 heating pad (fabric with a 100 watt wire heating element in it usually marketed as for using to keep pets warm or for treating muscle strains and the like) - you could sit that on an area of wood needing coating so that it's in contact with it, and it will also radiate the heat in, and that can be run for 24 hours for less than £1 without wasting energy on heating the garage. You could do this repeatedly to extend the first layer to cover a much larger area, and then subsequent layers can go on without the need to do any more drying.

I've never coated a boat in epoxy, but I do use it on the interior of wooden flutes (musical instruments). Yacht varnish works well too in flutes, and it's more civilised to work with as the fumes are a lot less dangerous. One of the problems with using epoxy in cold conditions is getting it to cure properly, and in these thin layers it doesn't generate enough heat of its own. Personally, I'd just coat any large areas with yacht varnish (I always go for the brand International, but ask around more widely for recommendations to see what's used most often) and save epoxy for places where things need to be stuck together hard and where holes need to be filled. Either way, you'll need two coats as the first always soaks in. The main effect of a third coat is to add unnecessary weight, but it may be worth doing this in the floor of the hull where water pools - that's the part that needs the most protection as it's usually wet much longer than the outside of the hull. Sand lightly between layers to avoid removing too much - the aim is just to make the surface nice and rough on the microscopic level to enable good bonding between layers. For the outside of the hull, again I like yacht varnish for the first layer to give the wood strong protection, and then put paint on top of that.

Incidentally, whenever I use epoxy, I now use a brand called Craft Resin - it's designed to be non-toxic once solid and the fumes when mixing it aren't too dangerous. It's used for arts and crafts in the home, but I now make flutes out of it and it's ruddy tough - you could use my flutes to hammer in nails. (I reckon you could make an entire boat out of it too, though that would be expensive.) That's what I would use to coat the floor (on top of the varnish) and a couple of inches up the sides, but I'd wait for the height of summer before adding that layer.

The instructions on the epoxy should tell you the minimum temperature that that specific type requires. If you have a garage that can be kept sufficiently warm and free of damp, that makes things a lot easier. There may not be a better time to do the job if the temperature's mild now, so crack on with it.