£100K ($142K) take home => £66,689.16 ($94K)
£120K ($170K) take home => £74,289.16 ($105K)
(£74,289.16 - £66,689.16) / £20,000 = 38% for keepsAlso the UK has very generous matching for pensions
For most 40% tax bracket earners (earning less than £100K) once you get beyond your employers match, there's not much benefit to a workplace pension. A £100K earner sacrificing 10% in to a workplace pension scheme is only £200/year better off than someone taking home that 10% and paying it in to a self-managed scheme (provided they reclaim the tax).
Things change dramatically over £100K. Someone earning £120K and sacrificing £20K in to an employer scheme ends up £4400/yr better off than with a self-managed scheme.
You get tax relief at your rate of income tax, which is much better for higher rate tax payers