Banks can't use your credit rating as a reason not to give you a basic bank account. However, they don't have a legal obligation to provide you with one either.
They'll normally prevent you from having a bank account if you're an undischarged bankrupt. I.e. you've only just Sequestrated yourself. Or if there's any record of fraud on your credit file.
If you've applied for an account and a bank has turned you down, you need to make sure that they were assessing you for a basic account rather than a current account. However, as a leading provider of debt advice in Scotland, fortunately - Trust Deed Scotland has experience of this  and we can, therefore, recommend the best banks to open an account with prior to entering a Trust Deed.