That was my thought. Think it's a bit cheaper to notify first than to notify retrospectively but 2-300 sounds about what these things cost last time I looked.Not hundreds of pounds , last time I used our council it was less than £200 .
I don't think the OP will need a G3 installer to check if notifying to Building Control as BC would be checking? I think the idea is that a G3 installer might be prepared to pretend he'd done the work and fudge the paperwork accordingly if the OP doesn't want to do it properly through the BC route.
Sorry to hear the OP has been dragged into this situation unwittingly, but good to know there are no dangerous installation defects.