As others have said you will need to leave the Terminal unless you fancy sitting / lying around on plastic chairs in the check in areas. I recommend plenty of the Heathrow hotels with free buses to most. The Hilton T5 is nice but a bit further out, there’s a newish Hilton Garden Inn, Crown Plaza or Atrium Hotels. Hilton T4 is also nice.
If you prefer convenience but at a cost then grab an uber to Hilton T5. Use the Tube to get to T4 T3/T/2 from T5.
Overall I recommend getting your head down somewhere comfortable if you find what you perceive as good value.