Bear in mind that you have about 8K tax free, and 12%NIC below 42K and 2% over it and have some cost to actually get to the office (probably). If you use the 50K salary as an example, you will easily end up with less than 1/2 of what you earned in that extra day in your pocket.
You’re still better off – but work/life balance and all that, obviously every circumstance is different but I think the choice of an extra day off each week rather than going to work for essentially a 50% pay cut would be very appealing!
You could however be financially better off by dropping back to 20% tax if you have perks with your job – i.e. a company car/health insurance etc. A decent BMW or Audi @ 40% tax could cost an extra 2-3K a year in tax over 20% which wouldn’t leave much change from that extra day…. and if you had free fuel that would be an extra £700 ish in tax!