I just wanted to let people know that if you have walking ability you have no need to be worried about getting on the Tube.
I travelled from Charing Cross to Kings Cross and back last week. I knew I wouldn't be able to manage my luggage and stick on the escalators as my balance is poor but the staff were wonderful and helped me with my luggage right onto the platform. There are help telephones on the platform to request this when you get off the train.

Whatever you do avoid the rush hour.