I’ve just picked a random date in august (2nd) and it’s £913 on the outbound flight from LHR to DXB, premium economy is £1092. I really cannot believe these prices.

As for your question, if you purchase a higher priced Y ticket you’ll be near the top of the queue to be moved up to PE if Y is oversold (quite common on PE aircraft) but then the price difference between a Flex+ and PE probably won’t be that much.