I applied with CV/cover letter online. Not long after I took a mental maths test online, it was similar to described by other people - lots of questions, not much time. The trick is to practice and keep your cool. I passed the maths test and took an IQ test, it was a challenge but they give a good example to practice (I think?) and you can practice similar tests online. They booked me a flight to come for interview (flight and hotel were covered, I was collected from the airport). I went to an in-house day in the morning and then had an interview. It was "fit" interview rather than competency - why I wanted this job etc. The whole process took 4-6 weeks, they were very friendly and helpful. Everything was well organised.

First was a mental maths test of roughly same level as Optiver 80 in 8. Then had a 1-1 interview with HR, did not pass as had not researched company well enough.