I did not mean to say that there are no twin seats in F, just that single pax can enjoy solo seats.
On a personal note, I usually travel F on TG with my wife and we avoid the twin middle seats. the pair in the last row (row 3) are close to the lav and galley and noisy. The 2nd row is facing the table where the FAs put wines and other stuff, very little privacy. Furthermore the middle seats have less space than the window seats. 1A & K are great for couples. You can talk and have a lot os privacy/space.

On the FFP issue, if you fly very little on *A, you should not worry about one FFP or the other. You wont reach enough miles to earn a significant award. But all Asian FFPs, certainly TG ROP or SQ KF are not generous if you fly premium cabins, actually not generous period. I would strongly suggest that you bank on BMI DC. In F you will earn 300% miles on DC and 200% in C. Plus you can use miles cash and cash to get an award. Plus you can open a family account with your wife and pool the miles to get an award. As you seem based in UK, the choice should be obvious. In C each of you would get some 30,000 miles (sorry don't have time for exact calculation), so a total of 60,000. If you fly one segment in F, it is even better.
With 70,000 miles and £480 you can get a return ticket in Business class to China or Japan on *A airlines. I let ROP faithful figure out how this compares to the earning/spending rates on ROP.