My 25 year old MEM consumer fuse box is still in A1 condition and I've
only had two fuses blow during that peiod. However I do feel I ought
to update and fit some MCB's.

My MEM fuses screw onto the live buss bar and the other side of the
fuse takes the wire for the ring main or whatever.

Can I replace these fuses directly with a screw on MCB like say this
from ScrewFix...
MK 16A SP Type B MCB 6kA

....or do I have to replace the consumer box to one that just has a clip
on supporting rail with the supply main wire going in one side of the
MCB and the ring main wire into the output side.

T I A Chris.