How old are your teens? Many hotel chains treat 16 years and above as adults for room occupation purposes. Room sizes, in general, are a lot smaller than you may be used to so, with 2 tall teenagers, consider a serviced aparthotel to spread the load. Will also give you the opp to eat in some times and save money. AFAIK there are very few smoking rooms left in hotels these days. Air conditioning is not always available especially at the lower cost end or in older buildings but, apart from the occasional heatwave, we rarely need it. In our house open windows and a fan in the bedroom usually suffice!
Here is a comprehensive list from a frequent US traveller to London. He is a regular poster on Tripadvisor's London Forum