HSE are always popular, BOISET etc. well as long as you get the correct ones (i.e. they cover Pan European including Norway) but personally I don’t thing this or medicals make a jot of difference to Employers as they will put you through it anyway, and you will just have to sit about until the rest of your course do theirs, so basically a waste of your money.
Any technical course that keeps you current would be good (fibre optics, computer networks, etc.) as opposed to some qualification you got 10-15 years ago.
Practical experience will always be highest on anyones list, not courses/pieces of paper.