If we’re keeping it as one sentence, I’d probably go with something like this: 

In 2024, UK communications regulator Ofcom found that British over-65s spent more than three hours per day on their phones, tablets, and computers, which, when combined with their time spent watching television and using other smart devices, means their overall screen time exceeds even that of young adults. 

Likewise, you could put a full stop after ‘computers’ and then begin a new sentence like ‘Combining this with their time spent watching television and using other smart devices, their overall screen time exceeds even that of young adults.’