I wouldn't call a £1500 rear beam cost effective. Especially if its not ideal for what you want to do with it. Mark is a very competent engineer and i suspect what ever option he chooses he will be making himself so that gives him free reign over the design.

The MOT station doesn't care what design your rear suspension is as long as it in good condition ie no play in bearings and doesn't bind. There are plenty of cars with de dion set ups that pass mots every year.