You can send them whenever you feel they're big enough (although check with the abbetoir in case they have a maximum size, ours isn't happy with 'big' pigs) my saddleback cross was about 9 months when she went. I had been free ranging mine and letting them forrage for food with a supplement of pig nuts, if you're feeding more intensively they might be ready to go sooner.
As for getting the carcass to the butcher, check with your butcher - mine delt with the abbetoir anyway, so we were able to 'hitch a lift' as it were, with one of their scheduled deliveries. If your butcher is local the the abbetoir, this might be the same. If you're selling the meat I suspect you'd have to go down the refrigerated van route - but I'm not really sure on the legislation. Again, probably best to check with the abbetoir or butcher.
Hope this helps
Karen