'Grass fed lamb' means nothing, since all sheep are grass fed, aren't they?
At least beef has a premium that warrants shedding them and feeding them hay in winter, but sheep... don't think so. Besides 'spring lamb'... born in spring, on the table by autumn...
'Certified organic' IMO can be paying for a perceived guilt-lable. I'd prefer to buy 'free-range' when I can afford to. I.e. I do believe meat is better when it comes from an animal that has lived outdoors, and lived something of a normal active life. And I don't believe raising animals in what is essentially a factory (battery farming) is going to produce any product of note. It can still work though since average cooking-plans don't expect anything of great note.
But compare your typical Xmas 'Butterball' turkey vs something like a free-range Norfolk Bronze. So different it can be difficult to believe they come from the same creature just raised in different ways.