A lot of people are being directed to other services rather than A&E. My dad has a health condition that often results in him being sent to A&E when he is stabilised and then moved to the ward. When shielding and self-isolation began his consultant's secretary rang him and told him that he has to contact the ward directly and will be stabilised there instead of at A&E. From friends in similar situations I know a lot of them have had the same sort of instruction.
There are also people who should be attending A&E but are avoiding going, such as the poster yesterday who has been having a gallbladder attack since Monday and was avoiding going to A&E. The NHS themselves have issued official reminders to parents about seeking medical help for their children as people are avoiding it.
Schools, parks, clubs/activities, softplay centres, etc are closed so there are no injuries relating to those.
Pubs are closed so fewer alcohol related injuries. People are still drinking at home but with a finite supply, less opportunities for harm (e.g., dancing in tables), and less opportunity to get into fights.
The sale of illegal drugs has slowed down so less drug related illnesses and injuries.
Fewer cars on the roads so less injuries relating to road traffic accidents, both pedestrians and drivers.
Lots of people either furloughed or working from home so fewer workplace injuries.
It's a combination of factors not solely related to time wasters staying at home.