Man has one of the most robust security procedures in the world, bar none.
Only airport on Earth where security threatened me with Police when I insisted on drinking my forbidden 50ml of remainiing water post security, old guy was conviniced this Scottish bloke in front of him was drinking explosive and was going to self detonate. His explanation, I kid you not. This was T2 but T3 REALLY do have some of the worst security on my travels, not anecdotal and there is a pretty decent sample size to that opinion. Awful layout, and very "particular" staff who pride themselves on doing things the special "Manchester way", again their own words there. DfT rules and standards just don't cut the mustard for that crew....
Laptops come out of bags so they can be clearly seen amid cables, iPads and hard clutter. An item of clothing over the top is going to make what difference?