"5.8 Where you are unable to provide MCS or equivalent certification under clause 5.5, you confirm that you are satisfied that the generation asset has been installed by a competent professional and meets all required industry standards and guidelines. Octopus accepts no liability for any loss, damage or injury resulting from the installation."
which opens the door to DIY installs if you get them signed off by any sparky, or get the sparky to do the final connection (which you may need anyway for Part P purposes).
Payback time on a DIY install could be fairly short, as in my example:
More detail in this video: