Another idea for you-Underneath most of the esstern end of Hyde park is huge underground (but not far underground as its a single story) car park full of nice warm vehicles and I belive it is heated to a degree too. Other end of Hyde park is Serpentine ie large body of water normally warmer than land. So two large areas that could easily be plus 1 or 2 degrees above average ambient for 'tarmac' London which I think is typically colder than a grass surface as well so it could a sort of micro climate thing and as has been pointed out ehre fog is fickle beastie a degree or two up or down can make a lot of difference
Cool view though isnt it and one of those moments your are glad you had a window seat
PB