The few KB taken just after midnight is easy to explain.
Your phone uses data all the time, bits here and there, it's not a lot but it happens. If you are connected to a wifi router, your phone will use that by default, but otherwise it will use your mobile data.If you've seen a big chunk of data disappear, it's likely your phone has been downloading an update to its operating system, or your offline satnav has been updating its map, or a whole bunch of apps have been given permission to update themselves whenever they feel like it.
Data gets used even when your phone is in standby Some phones will disable wifi after a short time in standby just to save battery. If mobile data is on, they will use that instead, otherwise they will tend to just wait 'til there's a data connection.
The easiest way to stop all this is to turn mobile data off.
Any mobile data that you use in any given day is rolled up into one total that's added to your account at some time after midnight.
This all means you can have data taken off your allowance a) even if you aren't using the internet yourself and b) even when you think it should be coming from somewhere other than your limited and/or chargeable mobile data.
When you exceed your data allowance, any usage of the internet is charged at £1 per day (£1 per 100MB per day if you are on PAYG). This includes the background data your phone uses by itself.
It's not a scam - you've either
got a handset you don't need because you're not using data but the phone is
not got enough data in your package to cover your needs and those of the handset you chose