Am I legally obliged to provide Income and Expenditure info to debt collectors (UK)?

They have no legal or official standing. They are a business and they can either accept your offer or not. They must make the judgement from their own sources. You might like to tell them that the payment offer will be met without fail as it is within your means and in your interest to do so. The alternative is to tell them that you will file for bankruptcy in which case they will get nothing..