Going large then !! (well it's a good size for UK)
I'm UK too, South-West, with recent 195L addition.
Good to hear you are going live on plants, I think it's a better end result. Don't know about rooting in sand though, someone here is bound to have done it successfully. What fish are going in ? If you are going to get a plec (recommended) they seem to like digging the plants up in my tanks. I have ended up re-planting and anchoring in with rock or bog-wood, maybe that's the way to get yours secured down too