Bulb billed me before account started

Bulb have billed me before the transfer from my old supplier even took place.
The account was due to start on the 1st of December.
Bulb made a hash of the transfer and I discovered three days after it went live that I was still with my old supplier.
Even though Bulb have billed me over £300.
They have offered me a derisory £20 compensation and told me that the account will now go live three weeks later than planned.
Meanwhile I am now being charged a penalty rate with my old supplier.
Not a good start Bulb

What do mean when you say you are “being charged a penalty rate with my old supplier.”

I am now out of contract and I have been moved to a more expensive rate