The limit on this one if the fact that it can only be used by a website design house. The drawback here is almost nobody sells just templates and the design pros out there go for other names like 'sitedesign' or 'designhouse', etc. I mean look at me, we are planning on offering templates for beginners/small businesses but because we were looking for something unique to stand for a complete website solution provider, we went with 'siteoven'. For this reason i would say developing it would be better. The other thing here is that tempaltes alone do not fetch that much cash, so the buyers of this domain will not spend that much on it to begin with. Its a great domain though. Given all that, i say $100-$500.
Edit: damn, i am tired... or typing too fast... fingers keep hitting the wrong keys.
P.S. if you dont mind me asking what is the cheapest registrar to feature .co.uk?