im designing a mid/rear engine chassis at the minute but im wanting to keep the engine exposed at the back. no covers or panels, is this legal?

Your sva man will pull you on projections and sharp edges.
To get over it put 25mm square mesh over it for the sva (securely fixed).
I had to put a grill under my rear wing for mine to pass

I know Beach Buggies and the like run uncovered.But you'll have to develop some form of cover for SVA time.

Yes, as said SVA require an engine cover, however I believe MOT doesn't have any requirement so if it fell off afterwards...

Having said that, I've heard of plod stopping buggies for not having a cover, and while they couldn't pin anything on the owners it's an hours inconvenience+having everything else checked to the letter, so might not be worth it.

p.s. think of the effect of rain on an exposed engine, you'll have to spend lots of time sorting out weatherproofing electrics etc.