Partially reducing the capital element of your mortgage repayments – but not to zero, which would mean interest-only – is known as a part-part repayment.
This option is not available under the mortgage charter.
However, part-part does fall under normal mortgage forbearance rules (see our Struggling to pay guide for more on this). So, you could contact your lender to see if it offers this – though be mindful that forbearance can adversely impact on your credit file (unlike mortgage charter support).