Unfortunately aircraft changes can take place and who knows it may even change again?

The only thing wrong with 50B, whilst no one steps over them is it's at the front next to the loos. There is a curtain right there which obviously has movement near the seat and from what I recall didn't look the best seats. Here is an old photo of mine from a few years ago re 5OK.

Personally I'd go for 53 J/K or 53 A/B. If you want these then of course you'd have to pay if no status. I'm sure the payment fees are actually designed to put people off and leave the better seats for those with BA/oneworld status who have a better choice (as in 53 ABJK) in later/late bookings. You of course don't have to pay however at OLCI there will be a limited choice available to move.

Maybe worth considering paying out? (Row 53 AB or JK) and not paying for the way back and take your chances on the way back as it's an overnighter anyhow.

More info in the wiki re opinions.