UK contracts often state that the company owns everything developed "in the course of your employment". This clause sounds very innocent. But it's not. See [1]. Note that using you own equipment, in you own time does not save you! From [1]:
"If material which includes IP rights is created outside office hours and/or using the employee’s private resources this may give rise to arguments that the rights belong to the employee rather than their employer, but that is not decisive. The fact that work is done outside normal working hours does not necessarily mean that the work is not done in the course of employment as, for many employees, there is often no clear demarcation of the hours of work.
The key question to be asked is whether the work was the kind of work which the employee was employed to do i.e. whether it was within the scope of their employment. Could the employee have been ordered to do the work and would it have been a breach of contract for the employee to not do it? The terms of any contract of employment and job description will be relevant, however, these (and duties more generally) often evolve in the course of time and it may therefore not be appropriate to rely on them exclusively."
How common is this in the UK? It seems like every company has this clause. How is one suppose to do anything on the side?
References
[1] Russell-Cooke associate Emily MacDonald looks at intellectual property rights within the context of the employment contract (2020). Available at: https://www.russell-cooke.co.uk/insight/briefings/2020/intellectual-property-rights-in-the-course-of-employment-who-owns-what/ (Accessed: 12 May 2021).
I sought an employment lawyer's advice on this once (as part of a larger question). The scenario is heavily affected by common law. I am guessing (as this is HN) that you are working in technology. The way it was described to me is that if you work in, say, banking, and you make your own banking app on the side, this could be argued to be "in the course of your employment". If you made a video game however (or something else unrelated to the business), the law is likely to side with you.
My best advice is to be transparent with your (potential?) employer (assuming you have nothing to hide) and let them know what you do outside of work hours and see if they will amend your contract suitably. If not, spending £250-300 for an hour of an employment lawyer's time is worth it and is likely to resolve your question.