"The trouble now is the problem of 'soft justice' in the UK court system. Drunken passengers are a regular feature of the newpapers and get away with it to fight another day."
I can't say I agree, at my airline all of the incidents that we escalate to the authorities tend to be dealt with pretty severely, including some high profile cases. I'd suggest that your example isn't representative of the hundreds of these I deal with every year.
OP: Write to the airline first (likely the head of SEP, Safety Standards, or Security) and let them know, they'll look into this. If you find you're getting nowhere then by all means write to the UK CAA.
Don't bother with the DfT, they do write security policy (note, not safety policy) but it's the CAA who enforce it. Disruptive passengers are one of those areas that involve safety as well as security so it'll certainly be the CAA who will deal.