5,500 zl gross.
Too little to have much of a life in Kraków. It should be about that much NET.
the salary is much higher but then so are living costs.
Despite the lower wages, living costs in Poland are relatively high, especially in the big popular cities like Warsaw, Wrocław and Kraków.
When making your decision, base it on how many POUNDS you can put away at the end of the month. Don't calculate your savings in any other currency, because it's meaningless. If it turns out that you can squirrel away 50 pounds a month in Kraków, and 200 pounds a month in the UK, then, regardless of the actual wages or cost of living, the job in the UK is by far the better option, financially at least.
Before I was living in Czech Republic as an expat for a British company but was earning less (this would be around 3,700 zl gross),
For that little, you would be reduced to dumpster diving and sleeping under bridges in Kraków, and probably also trading sexual favors to dirty old visiting German businessmen for candy bars and cigarettes. You'd squeak by with a lot of luck, little, if any, savings and no life to speak of at 5500 gross. To live somewhat decently you'd really need at least 7500 gross. And you don't become an "expat", rather than a simple foreign flunky, until you make substantially more than that.