From what you said:
I am a Software Engineer with around 5 years of professional experience and a B.Sc. in Information Systems located in Berlin/Germany.
Good, you have experience so you're already valuable - ie you're not a graduate.
In the U.K. (preferably Greater London), how do I find an employer that matches the following description:
Greater London is possible, but be aware that in areas outside London there is strong demand for developers. London has a lot of very good developers, so competition is fierce there - ie an employer outside of London has to give more to attract talent.
offers shorter working hours
See areas outside London
sees the benefit in an employee studying part-time
There's little benefit for a company to have an employee studying part time. Actually, there's no benefit. The benefit is having a better-skilled employee at a lower rate in the future, which is what your trade off would be.
ideally contributing toward tuition fees.
Yes, the tradition is they pay, and you agree to work for them for, say, X years (typically 2) after graduation. If you don't, you repay them the tuition cost.
Should you approach companies directly?
Go through an (specialized) employment agency?
No. Not unless you're very in-demand (ie a technical manager or you built a blockchain product)
Research the desired study programme/university first?
Sorry, what? You don't know what you want to study? What the heck are you going to tell the company "i just want to study something, will you pay for it?"
You NEED NEED NEED to know what you are going to study. Please stop all your other plans until you work that out. An MBA? A Masters? A Fin-Eng degree, a healing crystal degree - you need to know that. Not knowing it will make you sound like a lunatic to any prospective employer. You need to know the degree, and several target schools - as an example, some companies will sponsor an MBA, but only for a select few schools (Harvard etc). If you get into, idk, a regional school in Europe (ie not INSEAD) they won't support you.
Yes, what you want to find is possible, it will be harder to find in London, easier to find in the Midlands or Scotland. But nobody will just agree to pay your degree when you don't know where the degree is, what it costs, and what it is for.