I'm a mid level engineer earning £45k in Bristol, not including additional benefits and perks. To get that I moved from a small company to a very big software employer in the UK. I've found that UK salaries have a massive range but it's pretty much the big companies paying the top end ones, and then all the rest paying the lower ones.
I have many friends with the same years of experience as myself, and similar tech stacks. They're working for smaller organisations across the south of England and on roughly £28-30k with few/no perks past the standard UK pension payments.
I think a lot of people are underpaid in the UK (including the parent poster), in particular those at smaller companies and smaller consulting agencies. However I imagine it's less of an issue in London.
I have 4 years experience and earn £ 45k as an AI dev in a small business who sponsored my work visa. Secured 3 job offers in the same magnitude in diverse roles during my job search spree earlier this year. DevOps, Python data consultancy and this one. I have recruiters bugging me every day because of my polyglot experience but those roles won't sponsor my visa.
On QoL, I have a 2 bedroom flat in a nice area within a 15 min walk of tech employers. This is equivalent to owning prime real-estate in San Fransisco.