Am I allowed to wear shorts at work? —UK—?

I am a electrician in the UK working on MoD sites. The main type of work I do maintenance, like lamp changing, testing and minor new works. I have been told I’m not to wear shorts, only work trousers, even if all other people working in the building around me can wear shorts. When i was back on building sites, shorts were ok to wear. Are there any dress down policies for hot weather. My company just tells me about the risk of dermatitis, is this a big risk?. What about heat exhaustion, dehydration, heat rash etc. What is the greater risk? What would a risk assesment come up with for hot weather working?
Post people and police have dress down policies.

Thanks for looking, I just need some facts before the hot weather comes. Last summer was my first summer in trousers!

Wayne,

brighton electric