There is no false advertising. The £20 goodybag has a restrictions which are clearly stated on the website, one of which is that if you abuse the data allowance and do not alter your use of the service when requested to do so, they will refuse to allow you to purchase the goodyag in future. I am sure your solicitor will advise you that giffgaff are quite within their rights in doing so.
You may also be advised that you are infringing giffgaff's terms and conditions, which state that use of their services must be for your private, personal and non-commercial purposes. I am pretty certain running a one million plus subscriber Youtube channel would not be deemed private, personal and non-commercial use.
Turn off recurring goodybag and auto topups, and remove all payment methods [link]
Taking your number with you:
Dial 43431 or go to http://giffgaff.com/help/pac-code-request to request a PAC (Porting Authorisation Code)
The PAC will be delivered by SMS within 2 hours and displayed on your My giffgaff [link] page, under your number.
The transfer must be initiated within 30 days of requesting the PAC, otherwise it will expire and you will need to request another. Please be aware that mobile networks are not obliged to provide a PAC once your account with them has been terminated.
The moment you port your number away your giffgaff SIM card will be deactivated and you will permanently lose any remaining credit, goodybag and/or payback on your account.