Toby at work quoted the following from the post:

" Similarly, Good Energy has also always held onto 100% of its Levy
Exemption Certificates (LECs) to ensure that the renewable benefits of
the electricity supplied are not double counted."

and then asked:

'I thought LECs aren't tradable?'

Yes, I thought so too. I looked it up on Wikipedia but it doesn't have a page on LECs. It seemed it used to be there but not anymore. The closest I could get was a small paragraph that seems to suggest that LECs are tradable.