It's painful to do a tech startup anywhere but in my experience, what London suffers the most from is the cost of living.
I spent the last 3 years working in Shoreditch and most of the startup founders I came across were either bootstrapping to the extreme (spending nothing but sweat capital on their project) or freelancing at least 2-3 days a week to cover costs. I think in practice this makes a lot of the work look more amateurish in comparison to seed/angel funded teams in the US. In practice, you would see a technically very talented founder without the right UI/design, a visually polished MVP with no user traction, a businessy founder outsourcing technical work to East Asia etc.
I spent the last 3 years working in Shoreditch and most of the startup founders I came across were either bootstrapping to the extreme (spending nothing but sweat capital on their project) or freelancing at least 2-3 days a week to cover costs. I think in practice this makes a lot of the work look more amateurish in comparison to seed/angel funded teams in the US. In practice, you would see a technically very talented founder without the right UI/design, a visually polished MVP with no user traction, a businessy founder outsourcing technical work to East Asia etc.