I stayed at Royal Garden Hotel in London and LOVED IT!!! It's off Kensington High Street right on Hyde Park and next to Kensington Gardens. The breakfast in the morning was amazing. It's also really close to transit, the underground is just a short walk away (5min max) and there is a bus stop right at the entrance to the hotel.
In Paris, I really liked Hotel Muguet. It's in the 7th and some rooms have views of the Eiffel Tower. It's also super close to Rue Cler. Skip breakfast and just op for something from one of the shops on Rue Cler.
I really loved both Paris and London. Have fun!!!!