For low-skilled workers, starting cashier salary in Lidl and Biedronka (two largest store chains) is 2400-2800 PLN/mo [1], which turns out to around 7000 EUR/year. Same position in Tesco pays 14000 GBP/year [2] ( 16500 EUR/year) - over twice as much, but if we take cost of living (groceries, transport and especially rent) into account [3], difference becomes negligible:

700.00€ in Gdansk/, Poland in average has same local purchasing power as 1,257.65€ in Birmingham/, United Kingdom (assuming you pay rent in both cities). This comparison assumes net earnings (after income tax).

/* I used second-largest cities for comparison, since you know how out-of-touch London is, and Warsaw skews data in Poland in similar way.