If you earn £19000 before tax, are 21, still live with your mum and pay for all the things you need "extra" apart from rent and utilities...? (and live in London).
19000 is more enough in your case
Bit of a stupid question really as we don't know how much your overheads and outgoings are!
You should be able to live well off that amount if you are living with your Mum, but that all depends on what other bills you have such as credit cards or hp payments.