Just because your on the inside lane an the car in the lane to your right is going slower you cannot just overtake it on the inside !
Highway Code
163
stay in your lane if traffic is moving slowly in queues - if the queue on your right is moving more slowly than you are, you may pass on the...
No it is illegal to do it anytime, you can only overtake on the left only on a one way street or in slow moving traffic, ie queuing tragic.
Police do prosecute if they see you doing it.
Out of interest how long ago did you obtain your licence?