If you are going to build a Tenby then I don't think you will do better than Model Slipway. I'm not sure what comments you've read, but most kit builders on here I think would agree that Model Slipway are about the best UK kit supplier. Quality is good, support is good, documentation is good, particularly the newer kits.
I haven't built Tenby, but have built other MS kits.
Barrie