I looked in to this awhile back as i was looking at raising for the christmas market (i'm very happy to be corrected and welcome it). As i understand it you can home slaughter for your own consumption only, or close friends/family if not for profit (tracability of the meat). If you are going to sell to a third party then you must use a registered abatoir to kill it, you can then dress it for sale yourself if you hold all the relivent food hygine certs and the kitchen is signed off by local althority etc.
You can register as an abotoir but only worth it if you are doing alot. I have heard about more laxed laws if you slaughter less than a certain amount each year, i think the figure is 2000 birds but information is sketchy at best, talk to enviromental health at your local council, after all it is there responsibilty to monitor what goes on (mine lets me get away with egg sales to businesses so long as they are in the borough and i've not had to register as an egg packing plant

).
From my experience most abatoirs don't want to know unless your bringing the 200+ birds every week, however small free range producers that carry the relevent liecences and process on site will sometimes do the odd few here and there, but you will have to arrange getting the birds to them on their slaughter day and collect at a later date ( i must say though i normally get quoted £3 each so quite high).
Extra point to think about that triped me up is what to do with the guts as it can't be buried or go in normal bins it has to be speically collected.
Good Luck