depends where you are going

If you just stay in London and Paris, you do not need to join the tour. Either go yourself or join the local tour.

If it is your first time and you want both save time and visit more places, you may join the local tour like 3-7 days tour.

Don't suggest you to visit both London and Paris at the same trip since they are too different in culture, especially if your are first to visit Europe. You can visit France, Italy, Germany by the tour but if you will also visit British, it's too tight for you to visit France. Another reason is as my opinion, visiting continental Europe is more fun than British to understand European culture.