Also is it even legal for an under 18 year old to invest in bitcoin.
You are under the legal age and you are trying to start stuff in the
dark net?! You might want to slow down a bit, you don't want to ruin your future doing something illegal, do you?
I have already invested £20 which is now worth £40.
How did you do this last time? Why can't you do it again?
I think investing in Bitcoin is not an issue but using it for illegal purpose is an issue and must be avoided at all costs. There are countries where use of crypto currencies is illegal but it's illegal regardless of your age or race. So if crypto is not ban in one's country then he must fee free to buy Bitcoin.
I have already invested £20 which is now worth £40.
How did you do this last time? Why can't you do it again?
That's a good catch. If OP has already done that they why not again.