I am taking a United airlines flight from London to San Francisco (SFO) and connecting with a United flight to Honolulu.
Is 2:20 minutes enough time to make a connection in SFO?
Do we have to collect bags and re check in for Hawaii flight?
As it has already been asked in a comment, are the flights on one ticket?
If so, the airline gives you some assurance that the connection time is sufficient (if the flight arrives on time). In any case, you will have to get your bags after immigration and go through customs. If they are actually checked through to your destination, there are re-drop counters just after you exit the customs area, and the airline takes care of them to get on the right flight.
If your bags are not checked through, you will have to check them in again at an United counter, and then minimal check in times apply (how long that is can be found out on the airline's website). In this case, your transfer time may get a bit tight, although it depends on whether you are an US passport holder or an alien, and it depends on when your flight arrives and whether other flights from overseas have arrived just a few minutes earlier; I have been through immigration (not in SFO, but JFK) in 10 minutes, but on other trips, it took more than an hour.
My advice is to check with the airline and ask what happens if…