If you're willing to pay the change fee on your ticket (likely $250 + fare difference), you should be able to change to SYD-SFO-LHR-DUB or something like that. You could also indeed, as you said, book SFO-LHR as a separate ticket and just ditch the SFO-DEN flight.

You'll have to clear US customs either way, unfortunately, given we require even pax just transiting our airports do so. You should, IIRC, be able to check your luggage through to DUB from SYD even if you do the 2 ticket itin and ditch SFO-DEN, but you do have to ask them to do it. (I can't recall if you have to ask in SYD or if when you finish customs in SFO if you can recheck then to a different destination.)