£525 split between myself and my gf for a relatively large and relatively new 2-bedroom flat in North Kelvinside, just off Queen Margaret Drive. Internet and phone are £16 and the electricity can be anywhere between £60 in the summer months and £80/90 in the winter months. No gas in the flat, all electric. That's it for monthly utilities bills for us, but then there is things like TV licence, etc.
Having other people live: It's not allowed unless the landlord specifically gives you the authority to sublet part of the flat, but unless there are additional bedrooms in the flat, this is unlikely, and even if you do have additional bedrooms most landlords like to have as few people as possible living in their properties for the purposes of wear and tear, plus if they have more than a certain number living in their flat then they need to have a HMO licence and additional insurance, etc. So in most cases it is not worth their while to allow subletting.
If you were a single person in a two bedroom flat, or a couple in a two bedroom flat sharing just one of the bedrooms then the landlord may allow it if you ask.
You can't do it without their permission, although there is a requirement for landlords to give you 24 hours notice before visiting the flat for an inspection, so it may be possible to risk it and simply hide the additional luggage of the other person in time for the visit... although the neighbours may well notice the additional people if there is additional noise, etc, and if they complain then it's likely your landlord will terminate the lease and get rid of you.
It's really not worth it to breach the terms of your tenancy agreement.