HMRC give you from the start of the tax year April 6th until the 31st January of the following year to complete and submit your tax return. However lenders need an updated return after the 5th October so instead of having almost 10 months to complete it you have 6 months.
If the return hasn’t been done then you may not get the lender of your choice and the cost of your mortgage may be higher. Doing it early will also make your accountant happier.