I see you are in the UK. I'm not sure what is available for you over there or how it is packaged. I can just talk about things over here and assume it is fairly similar.
Chick Starter is generally a reasonably high protein feed, typically 18% to 20% protein. It's main purpose with the higher protein level is to get the chicks off to a great start, especially to help them feather out faster. Grower typically has a little less protein, maybe 16% to 18%. All the other nutrients are pretty much the same as Starter. Both of these have relatively low levels of calcium as growing chicks don't need it. Layer typically has protein levels around 16% to 18% and a higher level of calcium for the egg shells. The names Starter, Grower, and Layer are generic types, they don't really give you that much details.
There are other feeds too, a combined Starter-Grower, All-Flock, Flock Raiser, Game Bird feeds, and others but you didn't ask about them. As you mentioned, people do all kinds of different things. There are no hard and fast rules you have to follow. The chicken police are not going to take your chicks away if you do something differently than someone else.
As for your specific question, I feed a combined 18% protein Starter-Grower until they are at least 4 weeks old, then I switch to a 16% protein Grower. I switch to Grower when the bag runs out after 4 weeks. If it is at 8 weeks, that is not a problem, it won't hurt them.
I had never heard of the UK White Stars until you mentioned them. They appear to be specially bred leghorns. Since they have smaller bodies than a lot of our typical dual purpose chickens they don't need as much protein to grow and maintain bigger bodies. I don't know what protein levels you have in your Starter and Grower but I would get another bag of Starter to get to four weeks, then whenever it runs out switch to Grower. And, in the future when you decide what to feed them as adults, I'd not feel guilty about feeding them a slightly less protein feed than a lot of people on here use. With their smaller bodies they don't need it.