The china route beats mars hydro pants down if you know how to design it. And most likely cheaper.
But if you wannna go for a UK alternative i love these boards:
Benefits and Features High Efficiency lighting system Industry leading Samsung LM301H Evo 3.3 umol/J Full Spectrum Diffused White Light Diodes are combined with Deep Red 660nm V3 for improved efficiency and results. Improved PPE via 435nm wavelength UK manufactured CE, ROHS The ISH272R V3...
invisiblesunled.co.uk
But theyre quite expensive...
They have authentic samsung diodes, in lm301h and 90 cri, with red supplementation. They might be a bit too much in vegg but should flower like beast. If you put 4 of those on some 3 mm backing alusheet and put them on a hlg320-54AB you should be grand and have a light that you will never wanna upgrade. But it would probably be just over 400, maybe 450 with alu and cabling.
Diyled also have plenty of options and is help and sponsor here.