Collection rates are normally calculated as the amount collected as a proportion of the amount billed (or charged). The balance will be either still being collected or will have been written-off as a loss. The majority of income tax is either deducted at source (i.e. via PAYE) or self-assessed via income tax self assessment (ITSA), so is never actually billed or charged and as such a collection rate measurement is not appropriate.
HM Revenue and Customs does publish figures on revenue losses in its Trust Statement which form part of its published accounts, available at: