Correction: Braintree hasn't nailed it even inside US, despite reports otherwise. Even though you can sign up and get started with a developer account immediately, there is a human underwriter who looks at your account and activates production. This process might take lesser time now, but its not automated (like Stripe) and some people might get rejected on the way.
This step is the key value proposition for me. If Braintree or Paymill doesn't work like that, then they are not comparable to Stripe.
This is no worse then any of the other payment processors in the UK but hardly the frictionless option that is in the US.