We call price rises ~20%, but never have prices been raised by 20%, exactly, all across the board.
The price rise happens in different ways. Normally, box sets are rounded up, a couple of pound added on, etc. Paints might go up to £2.50 or £2.75... They may stay exactly the same. Elite infantry boxes in 40k, £20.50 at the moment... Might stay the same, might go up to £22, who knows?
So yes, anyone saying that prices wont be risen by 20% across the board is right. But the effect is pretty much the same. Prices will be about 10-20% higher if you buy the same stuff in 2 months time.