Haven't done more than 10 mins searching but Labour's line is that this was common practice for retiring DPPs, so they all had bespoke regulations passed for them.
https://www.legislation.gov.uk/uksi/2013/2588/contents/made
BBC is stating it is not clear if KS has saved enough to benefit from the measure.
There are a number of things here, not least this is being reported in the Telegraph on the same day that Sunak announces he has income of 58 billion trillion pounds. No coincidence.
Remember when Corbyn and Cameron released their tax returns? Corbyn has his salary and not a lot else, Cameron was worth more than many countries, and the story was the £80 fine (or so) Corbyn got for filing the return late.
If Starmer were a Tory MP this would not be an issue. Still, it does seem time and again the Labour Leader is always responding to negative stories and never out ahead of them.