But some Master's programs may require academic credentials. Degrees are also important for gaining work visas in many countries.
Finally, I feel like structured and more "formal" education would be very helpful to me when it comes to understanding math and more theoretical concepts of CS. I had no problem learning webdev on my own, but for more academic-type things, it would just be easier to attend a program that will make sure I have no gaps in my knowledge.
I’d love to do the CS course myself but £18k’s a substantial investment considering what I’ve managed to earn without the degree.
Aren't these points against your argument? That they had to change the policy to hire someone without a degree implies that it is very uncommon for someone without a degree to get into these upper-level positions to the point that they had never run afoul of this policy before now, or at least that they preferred to keep the policy rather than hire someone without a degree?
Remember also, particularly in IT, that educating yourself is going to be an ongoing task for you. It doesn't have to be formal but you have to keep up. Read Hacker News and High Scalability, when you see things discussed that you don't understand then take it upon yourself to go understand. I probably spend sixteen hours a week on my own research.