I'm not sure how you can shave 2k off the price, at the end of the day your only supplying the panels, and as far as I am aware installers aren't in a position to add £2k onto the price of materials but they will get them at trade price which is probably not the prices you are looking at online.
The cost of labour, surveying, scaffolding, certification etc will not change. You will save a few hundred quid by buying the materials for slightly cheaper, but you'll also waiver any warranty that may have been issued by the installer as they won't guarantee YOUR panels. Plus you'll effectively have designed the system yourself so no recourse if it under performs due to a mis match.
I really don't understand the logic personally, your looking at a 10%+ return and a pay pack of less than 10 years even if you pay for the installer to supply at the current prices, but you get a lot of protection against your investment at the same time.
The problem with solar installs is people forget that although the install seems relatively simple, it's the behind the scenes stuff like paperwork etc that takes as much time and people need to be paid for that as well you know because if your in the office filling out forms, your not out doing installs or surveying for other potential jobs. It's a business not a charity.
