What breed are they? If they are Super X Cornish or another hybrid meat chicken they are probably close to ready. If they are standard breed chickens then I would wait until they are 4-6 months so they can get some size on them.
I live in Herndon...not sure how close to York that is as I just moved here.
I will be doing some slaughtering within a week. It isn't as hard as you would think just depends on the method you use.