Coach
Coach travel is very popular from Luton, especially for passengers on a budget. It helps to travel light. We have used this and it was fine, for the distances involved, but very busy.
National Express operate a service to and from Victoria to Luton.
The fare starts at £5 or so.
The journey has stops in north London, eg Finchley Road, which may be convenient for you. Baker Street and Marble Arch can be stops.
Door to door can take a couple of hours.