What you feed them is very important. If you have British breeds, you might make more fat than muscle with high energy creep feed containing a lot of corn or barley. Just like us, high carbs/energy equals fat. When I had Simmentals, I used corn and protein supplement and weaned some smashing big calves come fall.
Higher protein (16-20 % depending on age of calves) helps muscle development and results in a solid calf without causing them to become lardy looking.
Some around here just feed 3-5 lbs./hd/day of rolled oats or mixed grain as soon as the calves will eat it.
I prefer and use a prepared, pelleted feed with a rumen bypass type protein. My calves are hard and thick at sale time. Because of their good condition, I think they should go ahead more quickly for the buyer and have a better reserve to draw on to get them through the transition to the feedlot.
However, I cannot say how much gain or benefit I get, because I have never done a side by side trial which is the only way one can determine the benefit. I do know that, comparatively, they have sold very, very well since I started on this creep feed and the same buyers are bidding on them as other years.
However, enough of the testimonial. I believe that there are some bona fide feed people or nutritionists on here who could advise you from a more qualified perspective.