Best practicle siteso we can visit London

My first post!

We are looking a taking our two teenage boys to see some of the capitals sites and sounds in the Easter half term. We would be looking for a convenient site off the M4 corridor as we are travelling up from Devon. Ideally somewhere close to a rail link to get us in and out of the city by train so we can navigate the underground as we select various  places to visit. Perhaps near the Thames for a river trip or two would be nice too! Where do you recommend ? Kind regards Fletch