There are a number of beautiful villages to explore in the Cotswolds if you have a car.
Try to avoid the weekend crowds if you can, but if you're around for longer, Stonehenge is on the way(ish) from London to Castle Combe.
Bath is a lovely Roman town and worth a visit! This could be a good option for one of the surrounding nights if you’d like to see more of the countryside.
Visit London to see the sights and say hello to Ush & Pip!
If you've got the time before or after the wedding, why not pop to continental Europe? Paris is a 2.5 hour train from London, and Italy is lovely this time of year!