It says 20-30 November to submit my first reading but yet the window for submission is still not opened yet.
I’m very keen to use Bulb asap.
You will switch to Bulb on the 25th, as advised. The date you give meter readings will not change when or how soon you switch to Bulb.
This confused me as well when I switched. Essentially what will happen if that the meter readings you give will be dated as taken on the 25th, no matter what date you actually gave them between 20th-30th. So if you give readings today (20th) you’ll pay Bulb kWh rates from today. But you’ll still pay for the next 5 days of standing charge with EDF until you officially switch on the 25th. So you’ll pay a kind of combined supplier tariff! I think this piece of information is what should resolve your concern?
You can always wait until the 25th to submit your readings if you want to be super accurate, it wont delay your switch.
But assuming the Bulb are cheaper, otherwise you wouldn’t be switching, it’s best to give the readings as soon as you’re allowed so that you can get an extra 5 days of cheaper rates until your actual switch date.