Bearing in mind the distribution in the UK of the Major Trauma Centres (only 26 of them), I'm sure you'd have to have evidence your decision making pretty well to justify bypassing one for one further away. I know of at least one incidence where this happened due to the helipad at the nearest MTC being occupied and the flight time to the next nearest was estimated to be shorter than the estimated time for the helipad to clear but I've not heard of it being done for specialist clinical reasons yet.
It's also worth remembering that some of the air ambulance doctors also have senior roles in the MTCs too so the 'us and them' isn't that clear cut.