What's a "Minor Works Certificate"? I personally can't
understand the need for any certificate about moving a
switch when coming to sell a house...

Well, it's free and hints that the electrician knows what he is doing,
rather than is about to appear on Rogue Traders. To fill the certificate in
properly, you have to do some basic safety checks, for example.

When present when selling the house it indicates that you took a cautious
attitude to electrical work, which may swing the deal if the people buying
include a panically paranoid type, of which there are many in the housing
market.

Christian.