Nick,
Interesting discussion. Long time since I operated in UK, but your comments regarding an "engineers" ability is well covered down here, where a modification of this nature would be cleared under CAR 35. Reg 35 approvals are delegated to approved engineers, who are allowed to sign out a modification after taking into account any stress calculations, electrical load analysis, etc., depending on the nature of the modification. Nonetheless, many of the initial design calculations for modifications on my aircraft are often done by our chief engineer, who then has them checked by a Reg 35 signatory.
Usually adds about $3-600 to the job, even when the modification is paperwork only, but it has streamlined the system and taken the onus away from the regulatory authority (CASA) and allowed more "hands on" approach to local modifications and fixes.