I think software is a little different than other tech/science companies.
Typically they need less upfront investment ( unless you go the buy customer route a la Uber etc - but the jury is out on whether they will ever pay back their investors ), and can grow much quicker and so are less constrained by institutional investors.
London is certainly very good for things like Fintech which employs a lot of software people on good salaries in London. But then city investors understand Fintech.
My experience is mostly hard science tech, rather than software per se.
I'd consider quite a lot of the software 'tech' companies to really be business model innovation companies rather than 'software' innovations ( ie software that's really hard to write ). The tech bubble classic example in London was lastminute.com - a holiday clearing house website - who's founders didn't understand the technology side.
One technical area the UK is strong in is ML - Googles deepmind started in London, and machine learning for Apple, Amazon, and Microsoft all worked on up the road in Cambridge. Again seeded by small companies bought by larger foreign companies.