fairly sure that london buses all have open-able windows
tube tends to have windows at the end of each carriage that let air through
this suggests about 90 minutes from waterloo to wood green on the 243 at peak times
yes, was going to suggest that.
blackfriars station has an entrance on the south bank now so not that far from waterloo (although may be further than you're comfortable walking at the moment - alternative might be waterloo east - london bridge and pick up the thameslink there, although to be honest it's a fair trek from waterloo main to waterloo east.)
or stay on the thameslink to alexandra palace station (think you would have to change at finsbury park, but still on the thameslink / great northern bit) then either walk or W3 bus for a few stops down the road?
aware you're outside london, take it you know that buses in london don't accept cash payments - you need either oyster card or contactless payment card now? (some people still get caught out by this)