If these are for your own freezer, or selling as boxed meat, you'll get more meat if you can leave them into next summer. But be aware you can't call it 'lamb' past their first birthday - it's hogget. In my experience, it's better meat - more flavour, but still cooks like lamb.
If you need the space, or need to sell it as 'lamb', or can't manage them over winter, then they may well be ready to go by end Sept. The meat will taste and feel different if you have to feed a cereal feed, and they'll start to lose condition on grass in October / November, so I'd suggest it's best to either get them off before then, or keep them on to re-fatten and fitten on spring grass. (And/or you can always feed a grass pellet rather than a concentrate, which affects the meat less, I believe.)
As to butchering. There's a good guide to the parts of the carcase and the choices
here We are a community with 20+ adults and 10+ children, so we use a lot of large joints. We eat about 12 animals of that sort of size per annum, and always love to have loads of burgers, sausages, mince and diced meat. A cutting list for us for 6 lambs / hoggets (of that type) for ourselves would probably be :
- 3 whole legs, bone-in
- 3 legs boned and rolled
- 3 legs as chump / leg steaks
- 3 legs for diced meat
- 2 whole shoulders
- 2 shoulders boned and rolled
- 8 shoulders for diced / minced / burgers / sausages
- 3 scrag ends sliced bone-in (for stewing)
- 3 scrag ends for diced / minced / burgers / sausages
- 4 best ends as cutlets
- 2 best ends for diced / minced / burgers / sausages
- 2 loins boned and rolled
- 4 loins as chops
- 6 loins for diced / minced / burgers / sausages
- 6 breasts boned, not rolled
- 6 breasts for diced / minced / burgers / sausages
- livers sliced
- hearts whole
- kidneys whole
- 80 burgers
- 10kgs minted sausages
- 10kgs spiced sausages
- 10kgs diced meat
- remainder minced
When we do surplus lamb / hogget to sell as a box of meat, we do something like this:
- one half-leg joint boned and rolled
- chump chops / leg steaks
- two shanks
- two half-shoulder joints bone-in
- cutlets
- loin chops
- one boned, rolled and stuffed breast joint
- 1kg burgers
- 1kg minted sausages
- remainder half diced half minced
(Offal not included unless requested - you may have to fetch the offal the day after slaughter, whereas the rest of the meat will hang for 5-7 days)