If one wishes to shorten the Tax year before the end of the tax year as per HMRC records.
It can be only done by calling HMRC.
It takes 15 working days to be completed at HMRC end.
Please note in case you are closing the company, please specifically ask the HMRC person you are speaking on the telephone to make the company dormant after the shortened period end date, otherwise HMRC will expect tax return for period after this date.
Also remember to change the `accounting reference date` in Companies House records.