You could probably get away with it being free if you drain the fat, and if it doesn't affect your weight loss great, if it does just don't do it again!
Just a suggestion here, but maybe you could keep a packet of Quorn mince in your freezer as a standby, just in case you can't get your lean mince at some time...
I use the lean and drain it, but i also often mix the smallest packet of lean mince with a pack of quorn mince..it tastes practically the same, and means that when i make spag bol/cottage pie there is loads so i can freeze portions for another day
As others have said I dont think it will harm if you drain the fat off
What I tend to do a lot now is use half the mince that a recipe requires and for the other half top up with mushrooms, courgette, carrot and onion, I find I dont even miss the rest of the mince and its cheaper to