Are you still paying your creditors? If so, then stop immediately and use this money to save for the fee.
If you are not paying creditors and cannot afford the fee then the CAB may be able to advise of other options. Some utility companies are able to provide assistance and if you are ex-military then SSAFFA may also be able to assist.
If these aren't options and there is no other way to save the fee then if you have available money on an overdraft or credit card this can be used but using credit to declare BR should be seen as a last resort.
If you have no available credit then it is a case of saving as much as you can each month until you get the fee. Some people have posted on this forum before that are in a similar situation and it has taken up to 12 months for them to save the fee.
You could wait until your creditors declare you BR but unless you are in debt with the Council or HMRC this is unlikely to happen.