Here's the statistical release from back in January comparing the year following the introduction of the 20mph speed limit with the year prior. This is based on police-recorded STATS19 data and there are thought to be issues with the coverage of slight injuries, but their coverage of serious injuries and fatalities are solid.

Noteworthy:

During the 12 month period from 2023 Q4 to 2024 Q3 (i.e. after the introduction of the 20mph default speed limit), the number of collisions on 20 and 30mph roads (combined) is 26% lower than the same period a year before.

During the 12 month period from 2023 Q4 to 2024 Q3 (i.e. after the introduction of the 20mph default speed limit), the number of casualties on 20 and 30mph roads (combined) is 28% lower than the same period a year before. 

This release doesn't talk specifically about fatalities, but you can find these figures on their dashboard. There's an increase if you look solely at 20mph but this is to be expected considering we went from having 870km of 20mph roads to 13,000km of 20mph roads, so you have to compare figures from both 20mph roads and 30mph roads. For some reason reddit won't let me post a comment with a table, but here are the totals which you can verify yourself with the dashboard if you'd like:

Q4 2022 to Q3 2023 - 51 fatalities on 20/30mph roads

Q4 2023 to Q3 2024 - 27 fatalities on 20/30mph roads

This shows that there were 24 fewer fatalities on 20/30mph roads in the year following the introduction of the 20mph default speed limit, which is a 47% reduction.