I would presume if theres mobile game companies in london then there would be demand for them. Presumably this was the type of company in that develop ad?
Also that company was probably offering that kind of money cause they probably decided that they would get one artist and pay him above the average but give him a load of work. Because its hard to find one pixel artist these days not alone a few. But im just guessing, but i would have though ordinarily it sounds a bit high.