Hi Ollie, like the others have mentioned.
1. take it slowly, don't get too excited People say they want a chicken then when you have them ready and ask that person how many they want, they say oh I'm not ready yet, can I come back in 6 months??
It's taken us 2 years to get to our little enterprise going. And yes, you will have to register with DEFRA and you'll have to go on the Poultry Register of Great Britain, which is run by DEFRA.
2. Yes, the tax man could be interested in you if you sell too many. So think about that.
3. Where is your start point? Are you going to breed your own? Raise them from day old (like we do) or buy them in just to re-sell?
Breeding your own, you'll need incubator(s) and then once hatched or bought as day old they'll need heat lamps for 4-6 weeks depending on time of year. Feed is not cheap, so you must work out roughly how much feed they will need until they get to point of lay, then double it, because ours are greedy little so and so's! Then work out the price you want to sell them for and then check on the internet or in local papers and see if you are charging enough, or too much.
Also like PG said, are you selling from your allotment? Most of them hae no selling rules which would count with chickens as well as veggies.
I'm lucky now, the only advertising I do is to occasionally mention on here how my latest flock are doing, plus I have a blog where I keep people updated, and some of my past customers come back again and recommend me.
So word of mouth is good, but you will only get that if your birds are of good quality. Many people actually come here and visit the chickens before they decide to buy, so always welcome people and tell them what you're doing. Then they'll know that you hae nothing to hide, unlike some peope we have experience of!
Can't think of anything else at the moment, apart from make ure you don't overcrowd them just to get more to sell, and like vember said, what if they don't sell? I think worry about that, but this last lost of Babcock's we have here are only 15 weeks old and most of them have gone, and the other 20 odd are reserved, and the ones in the brooder which are only 1 week old. 100 of them are already reserved for someone.
Hope this helps a bit.