The new £500,000 nil rate band for Stamp Duty Land Tax [SDLT] won't end on 31st March 2021, it will end on the 30th June. 2021. Then, to smooth the transition back to normal, the nil rate band will be £250,000, double its standard level, until the end of September 2021.
That means you will continue pay 0% SDLT on the first £500,000 until 30th June 2021 [Save £15,000]
Then from the 1st of July, you will pay 0% SDLT on the first £250,000 until the end of September 2021. [Save the 2% you would have paid from £125,001 to £250,000 = £2.5k]
From 1st October 2021, back to normal Stamp Duty thresholds.