Why would wearing uniform affect whether you get 1st class travel? Entitlement goes with rank, not clothing.
Uniform aside, the emphasis in MOD - and the 3 Services - is on cutting costs. Even though the functional standard says 1st class rail for sqn ldrs and above, if you insist on it for that reason alone you are likely to be told either not to travel, or to find a cheaper way of doing the journey.