The problem is the weather is so damned unpredictable in the UK. I can only spare a though for the poor guys working outside on Sub-Terra in all that gear right now. Its probably hard finding a costume that works with the overall theme, but is also functional.
I've always wondered this, but came to the conclusion that they are probably the luckiest staff. The uniform they have is most probably fully breathable and temp controlled, meaning they won't too too hot or too cold
As for themed uniforms, it's something Towers really need to invest in. The only valid hesitation I can understand as to why Towers don't so much, is staffing levels. Having a generic uniform allows the staff to work across all sectors of the park.