This is how it all went for us in the end. Checked in on line for Perth to BKK leg, went to Perth airport spoke to check in where we dropped off bags and got our boarding passes. They issued boarding passes for both legs. Perth to BKK and BKK to LHR.
I told check in desk that we were going to leave the airport and stay in hotel for the stop over. They checked our bags in for Perth to BKK leg only and we picked up our luggage at BKK and took it with us. I didn't ask about bags automatically transferring between Perth-BKK and BKK-LHR and us leaving the airport as we wanted to add stuff to our luggage that we were going to buy in Bangkok.
I went through passport control with my wife via the Thai lane I didn't have to pay any tourist tax or fill out visa on arrival form that I have in the past.
Next day just dropped the bags off at BKK as we were already checked in and had BKK-LHR boarding passes issued the day before.