What I would recommend might be different to your FE/HE institution. If you want to go and complete your degree then definitely opt for the Further analytical Methods unit along with the Advanced maths unit (something like units 35 and 61). Also the fluids unit, thermodynamics units and maybe the heat transfer unit. Design is covered adequately in core material.
The big fly in the ointment is wherever you study may put you off the maths units as their success rate with the 2 additional maths units will not be as high with some of the other units (they will be under orders to make sure completion/success is as high as possible).
As a little aside, the HNC is being downgraded to a L4 qualification, so doing the full HND (L5) is a good move.
Also consider Mech Eng foundation Degrees - there are quite a few FE/HE colleges in the North West that offer them and allow entry onto the final year of accredited BEng programmes.