Id go for comfy, you can wear all day, type shoes. Wouldnt want to be wearing flip flops in a crowd (not vvery comfy either). Trainers sounds like a good compromise. Maybe take you flip flops in you bag for when youre there and if it gets blazing hot and change into them for a while.
I live in flip flops though and used to wearing them in a very crowded pub!
however have been and brought some very cheap plimsoles from primark - if they get trashed it doesnt matter and a bit more support than flip flops
I cant face 12 hours in durbarry type boots on the tube etc and in this heat! theres just no need. we are on the count for how many people turn up in durbarrays and jodphurs!