As far as multiple hubs, it just depends on the specific model. In most cases Z wave devices and Many zigbee devices can be managed just with a smartthings hub. But anything else will probably require its own hub or bridge unless it’s made specifically for smartthings.
It’s not that big a deal, because generally you set it up with its own hub and app, but after that you can create all your rules (called “automations“ on the smartthings platform) and do your every day interactions through the smartthings app.
If you have neutrals at the switchbox you definitely have a wider selection of light switches to consider, particularly some Z wave switches.
See the UK lighting FAQ:
VFAQ: Lighting Control Options for UK SmartThings
Think of it this way. If you didn’t have a smart home system, would you expect your thermostat to also be able to turn on your entry lights? Or start your sprinklers? It’s not surprising that there are different control devices for different sub systems in the home. 
again, as long as it’s not something that you have to deal with every day, that is you have a unified interface for just turning devices on and off, The fact that there are different subsystems with their own individual controllers used when you first set up the devices doesn’t really affect you much.
So I think most people find that the important criteria is that there be a unified control interface, an “umbrella” interface if you like, rather than that there be only a single hub. 
Most people find a unified app is much more important to system satisfaction than a single hub.