it's not based on 18 weeks post birth, it's the 18 week period for which you are paid your PPL - and no, you can't return to work in that time
you can have "in touch" days, which are for training etc as mentioned, but to actually go back to work, no, you would cease to be eligible for the balance of your PPL from the first day you went back to work

if you were having training days, you would likely be asked for proof that it was necessary training to keep your job