One of the big advantages of redemptions is that they're flexible and refundable. It's such a nice novelty for me, it's almost (but not quite) enough to persuade me to buy the expensive (F, J) tickets in the future.

Now, it's in the very nature of flexible tickets that people will book them somewhat speculatively. I don't see anything wrong with this. I can see a case for charging a small admin fee, say £15 (or $20) but $50 seems a bit steep. Do they charge the same fee for date changes, which bring up exactly the same issue of taking up seats others could use? - particularly if you're changing from tomorrow, when it may be too late for someone else to take the spot, to a date in the far future. What about for cancelling a J ticket at the last minute?

Most people don't have enough miles to stockpile awards in the way you describe, any more than they're inclined to buy four J tickets with the intention of cancelling three of them. BA could explicitly prevent obvious cases of this, e.g. the common problem of the booking of three roundtrips from EDI-EZE in F with Tesco points, with overlapping periods.