It's not just software developers, generally people don't make much more or much less money than one another. There are exceptions of course but in general the low level job earns you the min wage and as your career improves you earn 1.2X, 1.5X, 2X, 3X and if you are a big deal you may earn 4X-5X and that's it as employee.

It's not even about the taxes and I don't think it's about the supply and demand too. In Europe, including the UK, there are not many high growth VC funded companies, so most companies operate on thin margins and there's not much room to wiggle.

That said, even 200GBP/EUR increase on your salary usually goes long way.