Pros
The interview process is far quicker than many larger companies and their technical tests are far better in my opinion. They give you a clearer understanding of what they are looking for in a candidate and they are aimed at assessing your actual programming ability in general, as opposed to the pure "implement this algorithm" style tests many other companies use which can tend to be hit-or-miss depending on if you've encountered the specific algorithm before. The work environment is friendly, there's free snacks and drinks in the office, and they make a conscious effort to avoid extreme crunch times. As of writing this I've only been with them around 5 months and it has already transformed my programming skills compared to when I started. This is mainly down to how helpful and willing to share experience everyone is here. I don't think I'd have been able to learn half as much in that time working anywhere else. If you're looking for somewhere you can grow your skills then West Pier Studio is certainly one of the best.
Cons
The commute is quite long if you live in London and they try to avoid full-time remote working. While this is not a problem for me personally since I moved down to Brighton and prefer working face-to-face, I can understand why it may be a deal-breaker for some. Although they can be flexible if you need some time to move down and they're very transparent with everything so it never hurts to ask.