In the end I went for the Infinikey Sactuary Rebirth PBT kit, it's the colour scheme I wanted and I love the gothic look. So now I have keycaps and will order the GMMK Pro, just need to decide on switches!Tai Hao PBT keycaps are pretty good too, although I couldn't find any sets I liked in stock or for pre-order.
There is a Mechanical Keyboard UK discord which has loads of helpful users too