Hi all. Newbie here. Kiltie is dead right. Do not give up your employed status. You lose a lot of advantages re pension, sick pay employment rights etc, also while it is easy to go from employed to self employment, it's not so easy going back. Get an accountant first, find one who specialises in small businesses.
Ask around, your plumber, shopkeeper etc. Also remember that working for one employer as self employed does not necessarly make you such in the eyes of the taxman. Your accountant will have no trouble sorting out your tax situation and in my own experience saves much more than he could ever cost. You are in what I always think to be an ideal position, a good job with a nice little legal earner on the side. Don't ruin it. Best of luck.