There are two companies that I am aware of who manufacture good kits: Woodies https://www.morriswoodwork.co.uk/ and Traveller Timbers http://www.travellertimbers.co.uk/. You will probably find that between them they supply most of the good parts suppliers, but will sell direct. One will use only Canadian Ash for quality whilst the other uses English ash for the same reason. Talk to them and make your own choice. Do not be tempted by bargain price/cheap "fantastic quality" "the best available" timbers. You get what you pay for.
I can only say that I recently visited Steve at Woodies, as I bought the traveller they restored and used at shows to publicise their wood work.
I was very impressed with the set up. The woodwork on the traveller I bought is absolutely superb. My father was a yacht chandler and I know a little about wood. The grain, finish. and colour of the wood on the traveller is beautiful.
I have no knowledge of the other Company, but I can thoroughly recommend Woodies through personal experience.
Ray Newell's book 'The Complete Traveller' has a whole section on wood replacement, and the car featured having the wood replaced, is the traveller I now own.
I bought my wood from the welsh company back in around 1999 or 2000. I think it was called ALT timbers from memory. The wood is still carefully stored as my 54 traveller project is still not quite ready for the wood yet! I bought it from there as I had heard good reports about it.