I agree with nmtg9 that the "pence first" idea is just a theory at this point, but I don't think it's important. The thing to remember with these is that the only difference is on the cover, even with the early L. Miller & Thorpe & Porter indicia changes, and that the interiors were all printed at the same time together. What order the covers were printed in is immaterial, really when you consider that the other 30-odd pages all came from the same pile.
Yes, colossus is awesome!, they are exactly the same as the 35 cent variants. In fact, many of the comics that were released in 35 cent variants also had pence variants in the same run.
A good question. I can only guess at an answer, but I suggest that most people prefer the more common cents covers because they simply don't know about (or know enough about) the pence variants. Speaking as someone with a foot on both sides of the pond, I like the added dimension these variants add to the hobby. I think the number of pence collectors will always be smaller than the number of regular cents collectors, but I've seen a real uptick in interest lately. I also think for very high grade key issues, the scarcity of pence covers can only drive prices up, eventually.