You are unlikely to get pulled up for a slight limp at the abattoir, especially as it will probably not be noticeable as they run off the trailer. What would be a no-no is transporting an animal that cannot weight bear on one leg. In the motion of the trailer, that would clearly be cruel.
You are best placed to judge whether your lamb is fit to travel. From how he uses - and doesn’t use - that leg, imagine yourself with a leg like that and the pain he experiences, and think about being in a swaying trailer for 25 minutes with nothing to hold onto and not being able to see the road or anticipate the movement. If you wouldn’t like it, don’t expect the lamb to do it. (He will probably lie down for most of the journey, especially if you give them plenty of straw, but you can’t ever guarantee that they’ll be sensible!)
If you do take him, be extra careful about bends and stops. Drive as though you have a pail full of liquid slurry in the passenger footwell!