I second the Victoria Line to Oxford Circus and change to Bakerloo line because you just walk across the passageway to change platforms - no stairs or anything. There is a lift at Kings Cross that gets you to the underground barrier level but you do have to take the escalator down. More escalators at Waterloo.
It is not possible to do it without changes.
Easiest way with luggage is to get a cab but that costs around £10, more if traffic is slow.