Hi, this is Jim, thank you for the question - I will do my best to resolve this quickly for you.
Sorry to hear of the issue.
Given British Gas moved you on to this tariff without your permission, you should make a formal complaint and if that does not resolve the matter ask them for a "deadlock letter", which is a letter giving their final response. The complaint to British Gas can be made here: https://www.britishgas.co.uk/complaints.html
Once you have the deadlock letter the next step would be to escalate this to Ofgem (their regulator) - they will investigate and liaise with the energy company which would hopefully result in a cancellation of the high tariff. They can order the energy company to make a financial award for inconvenience if they have acted poorly.
The service is covered by the Consumer Rights Act 2015 so you have a right to expect a reasonable service which is carried out with care and skill - if you do not then the regulator will step in. You can complain to the Energy Ombudsman by email here: ***@******.***
Or via their website here: www.ombudsman-services.org/energy-complaint.html
You should refuse to pay at this stage. Certainly until the Ombudsman makes a decision. British Gas should not take any enforcement action either. It is possible they could instruct a debt collector but you can turn them away - they have no powers as they are not bailiffs.
Ultimately if the Ombudsman does not uphold your complaint, and if British Gas sues, you will receive court documents (a response pack) which you must complete and return to the court. It is a tick box exercise for the most part and there is a short section to write a defence which is easy enough though please feel free to come back to this site if you need any more help. A claim will also take 6-12 months to be decided at court. If you lost then you would get 14 days to pay the judgment before the claimant can enforce the order, and 30 days to pay in full before it is registered with credit agencies. The claimant cannot recover legal costs if they win, in a claim under £10K (a small claim), all they can claim are the court fees and interest.
I can assist you going forwards if it gets to the point they issue a claim. I hope this helps and answers the question which is easy to understand - please feel free to ask me anything else. I will reply as soon as I can to help with any further queries.
Thanks for using our services and have a good day,