These are just educated guesses as I don't have particular experience with these aircraft, but based on my recent trip I would say
LHR 747 config 3 & 4 seats 32DEFG
The A346 seats 18AC & 18HK and possibly 18DFG (not sure about the bulkhead wall in front of these)
Just out of interest is there any benefit going PE? Apart from the better food (

) if you booked economy you'd get extra leg room due to the bulkhead seats.
Also if you've phoned up and pre-booked the sky cot then don't bother trying to go on and reserve your seat as the bulkheads are locked-off anyway. In fact that might be your answer - go and view the available seating and see which ones are locked off (but don't reserve any).