Gavin,
DfT screening guidelines state that electrical items and liquids cannot be covered up/in bags, and putting jackets back over laptops mean that you may as well have left your items in the bag in terms of screening, so, I'm not surprised they asked you to remove your suit jacket from over the laptop, it's something all airports should be doing.
This also ties in with the laptop ban due to a credible threat so while some countries have electronics bans inbound, outbound UK subject to extra scrutiny so cannot be covered up.
Just to show this isn't just a MAN issue, a quick Google shows that NCL/LGW and LHR all follow the same guidelines according to their websites. Whether they do it in reality is another question and pints to lax security if they are not doing so.
As said before, for a city that has had a bomb attack in the last few weeks with suspicions that there are more hidden cell members in the area, it's more surprising and frustrating that people still see fit to blast security as 'too inconvenient' for them.