You could look at its effect on currency in several ways;
1. The process of QE tends to cause short term inflation, so your £ is likely to be worth only 0.98/0.97 ish.
2. Or you could decide that the devaluation is best measured against the change in Sterlings value against, say the dollar, in which case it would seem that the announcement of QE2 caused the pound to drop against the dollar by a couple of cents.
3. Or , you could say that extra money has been created, so the devaluation is the amount of extra money created (£275 billion, QE1 +QE2) divided by the UKs GDP ( I think around £1.6 trillion) - who has the calculator?
Up to you which of those measures you wish to use ;)
Of course, as and when the economy finally decides to pick itself up out of the gutter, and hobble arthritically, every faster into the sunset, inflation should be curbed, and the govt bonds bought up by QE are gradually paid back, so everything should even out in the long run ( at least, thats the theory).
Jury is out on QE1 - i think it was felt that around 50% of the £200 billion got to where it was needed, the other 50% was subsumed in the extra inflation and banks bolstering their cash reserves...........
Thing is, QE is only going to work if businesses are crying out for money, which is being denied them by parsimonious banks - If on the other hand, business is just battening down the hatches and refusing to commit to spending whilst the economic forecast is so gloomy, it is difficult to see QE having any significant effect..... It really is all just guesswork and hope, at the end of the day...