Well it's horses for courses.
You say they are going to level the floor etc, but will it be as good as a flow screed floor finish?
For this, I would charge for priming the floor, then £25 per m2 plus materials nothing less.
Even if it was as good as a flow screed finish, I wouldn't ever tile / grout the same day - but that's just me 🙂
I also don't buy into the fact that people say, I'll only pay £12 per m2 or something stupid like that, because you'll tile and grout it in a day.
Would they pay extra if it took you 1 or 2 extra days?
I think not.
No need to rush any job.