I was at the same point as you until I discovered the new Andrew Ng course on deep learning [1]
It's a good structured way to learn the core of ML while learning about Neural Networks and without having to become and linear algebra expert which for most people including like me was a deal breaker with other courses. The timing is great too as ML now is so much different than it was 2-3 years ago.
The average is not a very useful metric since the gap between IB/hedge fund pay and the rest is huge. It's not unheard of for developers with 4-5 years of experience to make close to £100k.
That's just not how London works. It may be a cultural difference but a non-contractor candidate asking for salary during an interview is considered a big red flag.