rain will bring the temp down on any given day but that's quite a wide range for average stride of temp. It's far more common to see 20's than 30s. The tube however, will be hot and ridiculous... this will not change unless they replace the entire fleet of Tube Trains for new ones with A/C units in between now and 2 years time (a clue, this barring some new law enforcing it, will not happen)