Traf is a nice hall, and there is free non-traffic warden parking literally 2mins away from it, which is where I kept my car. Strictly you're not supposed to have a car in those halls, but the managers can never find out.
The place to park your car is in St James's Road, which is just to the East of Trafalgar hall. The end closest to the main road is blocked off, and there is a pay and display car park which you want to avoid, but loads of space on the road itself, and on Wellington Street which runs off it. Parked my car there for a year, never had a problem
10th Floor is all very well, but you'll hate it when the lifts break down and when you have an all too frequent fire alarm!