Wood is a solid Material, 1k laquer isn't. The epoxy might glue to the very surface, but in the end, everything ontop of the 1k laquer just floats on the painted object.
1k rattle can laquer is really not a good idea for basically anything that needs mechanical surface protection.

I thought epoxy had a certain degree of flex to it? How come it doesn't crack on materials such as wood that expand and contract?

The trouble with going over a 1k lacquer is that 1k lacquers never properly harden. So you go over the top of it with a nice hard 2k or epoxy or whatever, but the 1k coating underneath still moves around and shrinks and outgasses etc, and it effects the 2k coating over the top of it. So if you are after durability you are much better off removing the 1k coating and restarting.

Decided to go for 1k lacquer and it looks great but it just hasn't got the durability I desired and is scratching too easily

I now have the dilemma of what to do next. I have been advised a 2k lacquer won't go over a 1k lacquer which means i'd have to strip it back down and re-paint.

Would the XCR resin go over the 1k lacquer if keyed first?

Ok.. so the waveski is primed and paint is going on next, but i'm still unsure what to do about the final finish.

I spoke to a bodyshop and they wanted £100 to spray it with U-Pol 2k clear coat. There is a paint shop next door who could supply the exact same product in rattle cans for £15 (500ml), but i suspect this would take 4 cans.

A cheaper option is 1k lacquer which is only £10 a can, but this would be the least durable out of the three options.

The third option is the hotcoat (like in surfboard building) where a final layer of epoxy resin is put on. This would be less than £30 including delivery which makes the cheapest option and probably most durable, but most difficult to achieve.

Can anyone lend any further advice? Budget is tight so I don't think I could go for the professional spraying route.

Just out of interest, would the EC Glasscast resin work in this scenario?

You're much better off getting it professionally sprayed then, honestly. There no real reason you can't coat it with resin yourself if that's the way you want to go, but not only is it not an easy task especially on something the size of a kayak (you'll be up for many hours of sanding for sure), but there aren't a lot of options for brushable coatings out there that will be a good long term option. Epoxies are tough but aren't UV resistant, polyester flowcoats are ok but don't finish well and require a very good mechanical key to bond well... I also think you will add much more weight with just the resin than you might think.

A good quality marine grade 2k polyurethane is the way to go. Super tough and stands up great to weathering and osmosis, plus if you get it done professionally you will have a finish you will love. Go and speak to a couple of spray painters, there are always some that are willing to do a good cash deal and if you do all the prep-work yourself then it likely wouldn't cost you too much more than you would pay just for the materials to do it yourself like you are planning. 

Don't do the resin. Resin is very hard to get a good even finish with unless what you are doing is perfectly flat.

If you have the facilities to spray, spraying is far superior. Then you also have the option of buying pre-made metallic finishes. You can get powders that you can add to any resin or paint (look for PEX pearlescent powders), but they aren't easy to get an even finish with either. Stuff made for the job is the way to go.