The 88% is analysis performed by PHE, which showed that 88% of those who died from Covid did so within 28 days of testing positive. So let's say PHE's daily figures done on a particular day stated deaths from Covid were 9, the true figure might be 10. And don't forget that there will be some who died within 28 days of a positive test of something else, but will still be included as a Covid death.
Why do you want to see the figures on the old basis? They were becoming ridiculous and as time goes on and people die of something else will become even more so.