Very belated response, Tim. But do you need a 150mm reinforced concrete slab? I see from your sketch that it’s founded at much the same level as the walls, which presumably don’t contain reinforcement. I’m assuming you are not trying to span over old well shafts, or something.
Building Regs refer to BS 8103-1. Here are two snippets:
In other words, unless there is a special reason to beef things up, 100mm unreinforced concrete should be fine and would save you some cost and carbon. Worth noting that the limecrete slab of Ty-Mawr’s Sublime insulated floor is also 100mm thick. They had LABC Registered Details approval but that system seems to have been terminated. Even so, Building Control raised no objection to my proposal to use it my house.