When I was 17, I applied for a UK visa with my friend who was noted as my company (since she’s older than 18). On my visa, it specifically says that I can only enter the UK if I am accompanied by her. Now I have turned 18 and would like to travel to the UK alone. Would it be allowed for me to travel without my former company even though it says her name on the visa? Or would I have to wait till my visa runs out and apply for a new one?
You do not need to wait for your current visa to run out before you can apply for a new one. You can apply immediately; your current visa will be cancelled ("without prejudice," or with some similar notation indicating that the cancellation will not be held against you).
You would think that turning 18 would automatically cause the restriction on your visa to fall away, but apparently that is not the case:
What all of this amalgamates to is: don't do it, you could find yourself in detention facing removal.