Unhelpfully for you, we gas safe bods have been warned by GSR we must not commission and sign off new installations carried out by non-registered installers, on pain of being deleted from The Register. I'm sure there are plenty of GSR bods who would do it but they are taking a small risk that there is something wrong they missed, and could be held to account to fix an installation fault they've overlooked. Also registering the installation with Gas Safe as their own when it wasn't, is probably an offence of some sort too!