Started by filling the online application form + logical test (this was very easy).
After one month they asked me to come at the assessment center in London.
At the assessment center we started the day by a quite hard numerical test. Then we had to make a presentation about a case study (given the current trading system, how would you optimize it?). They give you 3 options to optimize it and you have to choose one and explain why you think that is the best solution. Then the group exercise starts (6 members in the group). You are given a role in the company and they give you a list of projects where you can invest some money. Together with your team, you have to decide on which project you want to invest the money and to give a presentation about it.
After that lunch (not too impressive) and then you have to give a presentation about one of your previous projects and about the solution to the initial case study about the trading system. After that a logical reasoning test (almost the same as the online one).
Then you have two interviews. If you wrote in your application that you have more than basic experience in a programming language you will have two technical interviews. If not you will have no technical questions. In my case I had two technical interviews. It's almost about what you wrote in your application. They asked me about OOP, about Java Garbage Collection, differences about Python and Java.