There's a certain number of seats on every flight that can be allocated to rewards - but as PE has the fewest seats of all the cabins, they tend to go early (as GK says). You can see what the availability is like by using the reward booking tool on Virgin Atlantic (same as booking a ticket for cash, but it checks the available inventory of award seats).

Impossible to predict whether the mileage requirements will be going up anytime soon, but as they only just got revised (mostly downward) in May this year, I'd think it was unlikely.

Pix.