(UK) What action can be taken against me for not informing my mortgage lender that i am letting my house?

I got a mortgage at 20yo in Jan 2008 and in April 2010, i moved in with my partner and began renting out my property through an agency. Only now has it come to my attention that i should have informed my mortgage lender of the change and altered my mortgage to a 'buy to let mortgage'. I decided to give them a call tonight and first of all query as to what the difference in payments etc would be. I was informed that i am not able to get a buy to let mortgage until i am 25 in just over a year. This ofcourse leaves me in a rather awkward predicament. Also worth noting is that, my partner and i have been thinking about adding my partners name to the mortgage. Their age is 30 so i think if i can add them on and then change to the buy to let, this would obviously be my key option. Obviously i am a little concerned about getting caught out and would like to know what action the mortgage company may take if they find out the actual situation. Please help, Thanks! Matthew