I don't know the answer to this but here are a few observations.
There's an electrical safety aspect to this. Is the installation correctly designed, installed and safe ? Get an electrical inspection done. End of issue.
There's a notification issue : solar PV installations up to 4kW are automatically permitted in most places but the power company (the DNO) have to be notified that the instalation exists once it's finished. Has that been done ? Ask them.
There could be a building regs issue - has the installation compromised the structure (changes to wind loading, added weight on the roof, compromised water proofing where the brackets for the PV are installed. I'm not aware that MCS approval or training covers architectural calcs so what value they can add here is unclear to me but look into this. It must be possible to get that aspect of the installation inspected and if it passes the electrical and structural inspections, then the mortgae and insurers won't have any reason to object.
There's a government subsidy angle - I think you can only qualify for the Feed In Tarriff (for what little that's worth these days) if your installation has been done by an MCS registered member using approved products. This may or may not matter to you. It won't matter a damn to the mortage company or insurers.
I'd like to have solar at my place, but I refuse to have cavity wall insulation retrofitted in the house. There are enough issues with damp as it is and bunging up the cavities might well make that worse. Without cavity insulation, I'll never get the energy efficiency cert that is also required as part of an application for FIT. So I'll never qualify.
On that basis, I have no motivation to pay a premium for a member of the MCS club to install some solar panels for me. I'm a fussy s*d and most (not all) trades have left me disappointed in the quality of their work so I'd rather fit it myself if I can get the install inspected for a reasonabe cost.
Of course the electrical work is subject to the regs and has to be tested and certified by someone qualified.
Why do I want it - even without the FIT subsidy it will be great to have reduced energy bills, free clean daytime energy to charge up the electric car or bike when these get cheap enough and reduce my carbon footprint.
