You make a reasonable point, but I think that there is something you are also missing.
I should make it clear that I am not an engineer, I am a wholesaler.
If you buy the boiler from whatever source (not us, we only sell trade) and then get a GSR installer to put it in labour only, thats fine, until there is a problem.
Who owns that problem? The manufacturer will claim faulty installation. The GSR will rightly say "not interested mate, I sold you xx hours of time for £yy". Call the manufacturer.
Answer - you own the problem. And putting it right will cost you way more than paying for a quality install in the first place.
But of course it may be fine. If so, you win. If not, you lose.