Ah, yes, the delightful "Biff and Chip" - all three of ours (not to mention their DPs) hated these books with a passion,
But much better than Ginn - that was really dreadful
When I used to teach KS, I used to have some children read almost all the books in a level (and some books make more sense if read in order, some it doesn't matter) and other children could just read a few in a level and them move on. Some children would skip through levels and then need to consolidate their reading by spending longer on a later level. A teacher needs to use their professional judgement.
Now I teach KS2, I had an irate parent complaining because I had told their child (a competent reader) they could choose any book from the last 3 levels. The parent paid the child each time they moved a level and was not pleased to have to pay 3 lots in one go!
Worry about reading levels generally comes from adults - intentionally or not. It is really important to show interest in the reading and none what-so-ever in the level / colour / box etc. And honestly who cares what level another child is on.