It sounds like human error on their part. However, there is a solution and you can avail of the Ombudsman scheme here - which should resolve the dispute.
It is free to use and quicker than court action - you have to first make a complaint to the energy company, wait for their final response and then go to the Ombudsman. The initial complaints should take up to 8 weeks and the Ombudsman can take up to 16 weeks for a decision to be made.
In the meantime you should dispute the bill. They should not take any enforcement action for the duration of the complaint investigation.
You must now 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 Scottish Power be made here: https://community.scottishpower.co.uk/t5/Complaints/ScottishPower-Complaints/ta-p/37
Once you have the deadlock letter the next step would be to escalate this to the Energy Ombudsman - they will investigate and liaise with the energy company which would hopefully result in a cancellation with no bill to pay. They can order the energy company to make a financial award for inconvenience if they have acted poorly.
Further, debt collectors are not bailiffs - they have no powers as such. They will simply try to collect the money their client thinks you owe. You can turn them away if you dispute the debt and they visit you - they can’t enter your home and take goods. They would report back to their client they were unsuccessful and it’s then up to their client whether to take matters further.
They will send a letter before action to you if their debt collection attempts fail - which is required under the pre action protocol to give you the chance to avoid court action. They usually give you 14 days to pay before they will take civil court action. I doubt they will do this until the outcome of the Ombudsman's decision is known and I believe the Ombudsman will uphold your complaint - which should resolve this for you.