Please read the
old rumours thread for details. I will summarise here.
These rises are coming straight from Pentax.
These are just the selling prices. RRP is actually higher!
The excuse given is the value of the Yen versus British Pound Sterling. While this can account for a significant increase, some of these lenses are going up much much more than that. So the given reason is not the entire truth.
Prices have also risen throughout Europe and in Canada. It's not just the UK, so it can't simply be about the value of the pound.
Some speculate Hoya wishes to price Pentax as a premium brand, no longer the value-leader.