It's a tough game I would have thought. Regardless of all the selling / stamping regulations, can you find enough customers locally? 300 prime layers are going to give you maybe 300 eggs each in year 1. So that's 15000 boxes of eggs, or 288 per week. Feed is going to be something like £4200 I would imagine (going on £6 a 20kg bag), or 28p per 6 eggs. I reckon on £14 per hen per year at those prices for feed. If you could achieve £1 per box average wholsale / direct then you have a margin of 72p *15000 =£10,800. Then you have to figure in replacement hens as to get 300 per hen per year you are going to need to change them out every year. So, say £5 per bird = £1500. Then there is the transport and time to get these eggs delivered. Wear and tear on vehicles etc.
I can't see there is a huge amount in it. Maybe you can achieve a better price where you are, I don't know. Good luck with it but I would do some very careful sums if I were you. The main thing that concerns me with going to that scale is the time to deliver all the eggs. I have worked it out for the local market and I can't see it working for me at that scale.
Keep us posted as it's an interesting concept.