All taxpayers who pay any income tax at the basic rate would benefit. A reduction in the basic rate would result in a loss of tax revenue which can be approximated from table 1.6 ‘Direct effects of illustrative tax changes’ on the HM Revenue and Customs website at:
http://www.hmrc.gov.uk/stats/tax_expenditures/table1-6.pdf
The figures exclude any estimate of behavioural response.
There would need to be a corresponding reduction in income tax deducted at source on interest.