Should I switch to flowering?

The only nutes I'm giving are biobizz grow, 2ml per litter

I can see clawing on some leaves actually so you might have an excess of N, but the low yellow leaves are usually signs of deficiency. Wait for someone else to help out before doing anything

The leaves look very dark to me, you’ve got some nute deficiency on the bottom leave there I believe. Would be helpful if you had an up to date diary. Doesn’t look like excess N so I’d lean towards are you using any calmag? If so maybe using too much . Magnesium will darken leaves like that I had that problem in my latest grow. Don’t switch until you’re sure the plants are healthy