I have some lengths of 50mm x 25mm x 1.5mm thick seam welded steel box
left over from when I built the garage doors. It's pretty stiff
(unlike angle) and probably stiffer than the support the cupboards can
provide?

25mm is, perhaps, a little too much, but pretty close. Although the
instructions call for 40mm wooden beam across the front, I am hoping that
doing several structs of metal 20mm will provide enough support.

I assume you wouldn't be able to take the box section along the entire
length of the worktop (this would provide more support?)

No. The next item is the dishwasher, which must be flush against the
worktop, unlike the washing machine and tumble dryer, which have the
facility to bolt the doors artificially high to cover the gap. Between the
dishwasher and tumble dryer is a stud framework with a pretty column pillar
on the front. The studwork is made of 47mm x 75mm softwood, so should
provide enough support for the metal lintels. I was planning to channel out
about 30mm for the lintel footings with a router.

Christian.