Are you talking about finance, or a regular purchase? If you make an up-front purchase of a whole system (in the pre-build section) then there are no hidden fees, they build it to spec and the build fee is included. The only possible issue I can think of, is if you get out of stock items, but you don't have to agree to swaps.
If you spec your OWN system from separate components then yes, they would charge you to build it.
If you're referring to finance purchase, then I'd check with OCUK directly.