I think the point here is that he describes it as a CAA permit not an LAA one (I know they are all CAA permits really). AIUI there is a CAA airworthiness inspector who will be designated responsibility for the continuing airworthiness of that particular type or individual aircraft. I suspect that person would have ultimate responsibility for sanctioning substitution of parts or equipment.
If you could get it transferred to LAA then the approval of panel changes would be something within the remit of an LAA inspector and would be a lot simpler (and cheaper).
In principle, you don't have to comply with a type certificate as you don't have a standard CofA, so TSO'd equipment is not required.