more secure running linux
While nobody would argue that out-of-the-box a linux distro isn't more secure than windows (maybe except for the Red Star ;) ) established companies have already invested millions in securing their windows-based infrastructure and microsoft hasn't been very keen in allowing their partners to interoperate with linux-based projects.
There were some success stories: internet protocols were made in such a way that you can put any computer (be it linux, windows or mac) next to windows and they can talk to each other mostly fine, which drove some innovation.
On the darker side in the desktop-space however commercial firewalls or antiviruses (not that you need an antivirus on Linux) need to be tailored-made for an operating system and that's burdensome enough for the inertia to keep taking its toll among the IT system administrators.
security and privacy issues on windows are caused by the apps themselves
that's true that proprietary software, which is mostly found on windows is riddled with bugs and security vulnerabilities, but it's also worth noting that windows architecture itself is just more "bloated" than that of linux (and other unix systems), which makes it harder to secure. "MAC" systems like SELinux and Apparmor are way stronger than anything windows has to offer. "Containers" introduced a few years ago made running apps in isolated environments (via programs like docker, firejail or flatpak) much, much easier. That's all possible thanks to good design decisions made decades ago.
companies usually want something else, they want mostly someone to call and blame when things go wrong
I'm really curious how many companies are there that could provide IT administrative services based on open-source software. You have giants like Red Hat, OpenSUSE or Oracle, but I'm curious how many "boutique-like" companies could be there. If by any chance someone is serviced by such a company around London area, please let me know.