Heathrow exit question

Hi!

My wife and I are planning a trip to the UK in march. Our flight is scheduled to arrive at 9am. We want to head straight to Oxford (probably drop our luggage at a luggage holding spot we'll find through an app). The worry is—we've never traveled internationally. We're looking at train tickets from LHR to Oxford station at around 11 am-ish. Is that enough time to get through customs? Thanks!