I think you may be mistaking the £150 CT rebate [not required to be repaid] with the £200 Energy Bill credit we will all get in October and will require to be repaid in four £50 instalments over the following 2 years - affectionately called the "loan, not a loan".
Not sure about opt outs but...
Not much help, I know, but ....
I'm under Telford & Wrekin for my local council tax and living in a band E property.
I was pleasantly surprised to receive a letter from them in early April saying they were going to pay a discretionary £100 to all band E's in the borough,
Happy to say this...