How's that? You net maybe 2-2.5k pounds per month contracting in Warsaw, and 8k for an equivalent job in London. Rent and commute in London is maybe 1k more than in Warsaw.
I get 1500 pounds net per month in Warsaw with 2 years of experience. For London (according to numbeo.com) I would have to make at least 4500 pounds net to have comparable lifestyle. That's 80k pounds per year before taxes - AFAIK not feasible for someone with my experience.