Ok, a couple of phone calls later...
I spoke to a guy who has a small business plucking and gutting, all properly licensed for food hygiene etc
mostly game/waterfowl but he has done chickens and turkeys before so has a fair bit of experience with it.
They need to be supplied to him already culled so you'd still be looking at getting trained and licensed to cull your birds, refrigerated transport etc etc
He's not really interested in doing turkeys at all, he's done them before but they usually need doing bang in the middle of the pheasant season which is his busy time.
Also, people want them hung for a week and they take a while to do etc so he has to charge £20-25 per bird whereas most people go to him expecting to pay a couple of quid per bird
He charges £2 to pluck and gut a pheasant, chickens depending on size/age etc would probably be £3-4 each so unless you are charging a very healthy premium thats your profit already gone.
His advice is to contact Environmental Health and look into getting set up for processing your own birds, its not as difficult as it sounds apparently.
I suspect thats not what you wanted to hear but short of increasing your bird numbers significantly it may be your best option.