I've noticed that the kits available over here are often very different from America.
Often the American kits will be glued up and floor tillered, so that the would-be bowyer is more or less guaranteed to get a shootable bow from their efforts. Usually there is some handle wrap material and a Flemish (or similar) string that can be adjusted or, if the bow is a pre-loaded reflex/deflex, it would ship with a standard double ended string of appropriate length. This goes for anything from selfbows to Howard Hill bows (which can be bought floor tillered for you to finish out yourself) and for a whole raft of various laminates from different craftspeople.
Over here many longbow kit sellers seem to offer a laminated sandwich and stop there. Or a stave...
Unless the kit is mostly formed I see no benefit at all to the end user, who would be better served sourcing a few straight grained Ash boards from a local hardwood supplier and make some selfbows for a fraction of the cost.
Basically if the buyer isn't much of a bowyer they need most of the work already done or they're going to screw up an expensive kit just the same as they would screw up a simple board or harvested stave.
If the buyer is a decent bowyer they will only want a kit if it offers them something they are not equipped to produce - reflex or combined reflex/deflex for example, bamboo backed, mixed exotic timber cores et al, all of which can be difficult to produce unless you are tooled up for it.
I have never understood longbow kits - although I have never bought one I mostly view them as very cost prohibitive to the buyer and, with most of the work done, all you really get from it is an exercise in final tillering - but that doesn't stop a lot of folks from buying them and building from them with varying degrees of success
You can find examples online from many UK bowyers, and on eBay, but to compete with those already established you will have to undercut prices or offer more for the same amount.
I recently bought enough Ipe and bamboo for 6 bows for the price of some kits out there. I will get six bows out of the material as well, and they'll be fast
Don't forget to read the rules and regs for selling on the Makers Market part of this forum, and good luck