Selling live wouldn't be a problem, if you could get enough for them to make it worthwhile.
Your 18p a Kg is for what sort of feed?
The benefit of commercial rations is that they are pretty accurately balanced.
Unless you know what you're doing nutrition wise mixing your own rations you can create an unbalanced one which they'll just eat more of to compensate.
If the bird eats 200g per day of a 20% protein commercial ration to get the 40g of protein it needs and you feed it a 10% protein ration it will just eat twice as much feed.
I thought about selling live turkeys at Christmas but there's hardly anyone nowadays who would know what to do with a live bird.
If you could find a niche in the market, a local breed free ranged and fed on locally produced organic foods etc you might be able to make a small profit but you'd need to charge a fair premium.
There have been a few people on TAS looking to do similar to what you want to do over the last year but as far as I know no one has found a processor willing to do small numbers (20-100) they all want to do the large jobs, its probably not worth their while to set up to do a small lot.
A game plucker and dresser I know said he'd pluck and dress chickens for £4 each but they'd have to be delivered already culled and in refrigerated transport which doesn't help, you'd still need to sort out that side of it.
His advice was for the grower to contact the council and get certified to kill and process themselves which takes you right back to the start.
Its just not viable small scale.
If it were I'd be doing it!