Thinking of visiting London with 2 DC. DD has wanted to go for ages she wants to see the queen ! 

I think she'd like the natural history museum too. However, DS is autistic and would probably be in the major buggy. (He's an on walker but not too safe and really heavy if he decides he wants to be carried...)
Is the tube impossible with the buggy ? What about busy places is it a potential nightmare? I don't want him to miss out but I'm apprehensive about getting around with the buggy.
Any advice or tips welcome thanks