Up here, butchers want larger lambs - 24-26kg deadweight, which with a good conformation Texel type will be 45-50kgs liveweight. The most popular lambs in the auction ring are good conformation Texel types which will kill out at 20-21kgs (so around 38-40kgs liveweight) as this is what the supermarkets want to buy. Smaller yet for the export market.
You won't know until you have a few of yours away, what conformation they have and what killout %age you get. On the moorland farm, our grass-reared Texels-out-of-mules would be mostly R3L (bog standard, absolutely fine, conformation - but not in the bonus bracket) and would kill out at about 40-44% of their liveweight. Here, BH produces top spec (conformation E2, E3L, U2, U3L) Texel types which kill out at around 50% or better.
You can't much influence the E, U or R by feeding, it comes mainly from the breeding. (Although an unfit lamb will likely be a point lower than its potential.) You can make them too fat (anything over 3L is too fat) with food, or too thin (1 is too thin) with lack of feeding.
A lamb is ready to slaughter when it is fit - condition 2 or 3L - irrespective of its weight. If you send them away because the weight is right but they haven't yet achieved fitness (CS 1) you won't get as much for them, and if you keep them longer hoping to get them heavier they'll just slab on the fat, be condition score 3H, 4 or more, and again you won't get as much for them.
So if it's about deciding if they're ready to butch, learn to condition score. Eblex have good leaflets, and cracking hands-on materials if you can find them at a show.