Your bags will be checked through to FRA and you'll be issued the LHR-FRA bp in HKG.
On arrival at LHR just follow signs to baggage claim and exit and have a nice day in London. There is no need for you to collect your bags at LHR.
Return to the airport in good time to get through security etc.
On arrival in FRA you'll need to go through immigration the same as everyone else from the LHR flight.