I’ve just booked this myself for November. I booked the outward flight as soon as it became available and then the return when that became available. So two singles. From memory the first flight was around £380 in taxes and the second about £30 or so, so just over £400 in total. The taxes from HK are much cheaper. I was checking everyday and there was always UC and PE availability when they were released. If you grab them quickly you should be fine.