I agree....you bloke's prices are very high. I bet you could buy an Ls1 with gearbox (or transmission, depending on what you want) from the States and have it shipped airfreight to the UK and still be quids ahead.
I think that you could find a decent, low mile pullout from a yard here for between $3k and $4k and spend about $800 to ship. THat leaves you another $2-3k ahead. You do not need an aftermarket ECU, unless you want all the bells and whistles, like launch control, NO2 control etc, but you can likely get standalone systems to do all that. The OEM computer can be tuned and editted easily.
Tim