Can I just ask why you need to return to the UK to sell your house? We had ours rented out for 3.5 years until we sold it last year, not setting foot on English soil. Our situation was different and we hadn't tried to sell it before (having originally left the UK with the intent to return after 3 years). The agents and lawyers were happy to deal with us from overseas. Our tenants stayed in the house until right before completion. We weren't paying UK or NZ tax on our UK rental income because the shared income didn't reach the individual threshold, but obviously with your pension incomes, that is likely to be different for you.
Our main residence is still in UK which apparently means we can still be classed as UK residents.
Can I also ask where you got this from? I don't know if it's right or wrong, but just because you own a house somewhere else, doesn't make it your main residence. You have to live in it for a certain period for it to be classed as that (I believe).