You will have so much fun!
Love ok into a food tour and maybe a cooking class
You can rent bikes and ride a it to one of the castles stopping at multiple plus for food.
There are really cool archeology tours where people hunt for treasures in the Thames (that’ may not be of interest to your 18 year old but mines would love it so mentioned here
There is the Thames Rocket tour which is a lot of fun.
There are tons of walking tours including a rock music tour (look it up on Viator)
And check out concerts for the dates you are there. Am sure there will be many to choose from!