Fascinating wiki
For example, using figures in July 2008:[5]
the price of a Big Mac was $3.57 in the United States (varies by store)
the price of a Big Mac was £2.29 in the United Kingdom (varies by region)
the implied purchasing power parity was $1.56 to £1, that is $3.57/£2.29 = 1.56
this compares with an actual exchange rate of $2.00 to £1 at the time
(2.00-1.56)/1.56 = 28%
the pound was thus overvalued against the dollar by 28%
And the GBP/USD currently stands at 1.566, which IME seems to be a fairly typical rate over the last few years [odd peaks and troughs excepted)