When I've looked at salaries in London they seem relatively low compared to the States. Perhaps higher isn't high enough?

Consider that senior developers are busy and aren't going to make time to prep for interviews that lead to a job landing them a $15k-$20k bump (before taxes). Together with all the other intangibles (culture, nature of project work, career paths) it makes jumping ship difficult to gauge.

Three suggestions - 1) raise the compensation for seniors 2) offer more interesting projects/products 3) use a different interview process for seniors, they hate algorithm quizzes

Most software companies just churn out database-backed MVC apps. After 5 years of that any good developer would be bored.

Have you tried offering internationally competitive salaries?

Then there is also the issue of the ease or difficulty of getting someone from a foreign country to work in the UK. Are work permits required? How much paperwork? etc

If I were an American, what would it take to work in London?

> If I were an American, what would it take to work in London?

A good internet connection and some perseverance in getting remote jobs. Also strong work habits.

I have many friends back home in Slovenia who stay in Slovenia but work for San Francisco. Sure they don't make 100k, but they don't have to. At 80k they're socking away 40k+ per year. And the SF startup is super stoked because they get good engineers for peanuts.

Well, why would those senior people want to apply and work at your company? This is not snark; this is a real question you need to be able to answer if you want a chance at hiring those senior people. Why would a senior person want to up and leave their position, which likely already pays pretty well, and treats them pretty well?