Comfort all the time. There is no reason at all to get dressed up on a holiday charter flight.
Some people get dressed up on Schedule flights, in hope of getting an upgrade, but basically this just doesnt happen on Monarch Flights. My dad did manage to get some jam or fruit sauce down his football top ( Leeds Utd) and decided to take it off. The Monarch staff kindly asked for him to put his top back on. They said it was company policy for people to have their shirts on, I just thought they wanted him to show off his Leeds top whilst we landed in Manchester.
Shoes- try to go for slip ons. Always best to slip them off. Travel socks recommended.
Coats in winter , just take a nice warm fleece. No need for anything else. I did need the fleece this year due to unseasonable weather around 10th feb.
You can always take a change of clothes in your hand luggage and slip into some shorts somewhere on the flight (or Bahrain). Or going back slip into your long trousers.