If I were just starting construction on your garden, I would suggest putting an inground pond in with waterfalls bubbling out of the wall. But your garden looks very established - I personally would not be happy pulling it all down and starting over.
At my former hillside home, the hill was terraced and I did have small built in ponds. But I also found some beautiful large glazed bowls that were meant to be water features. If you place a large water bowl where the Hebe is and added a battery or solar (I'm not sure how much sun Surrey gets) operated water fountain, you would have your feature.
If you find a big bowl you like but it has a drain hole, you can seal the hole. There are so many things you can do with a water bowl, including move it if somewhere in the future, you change the plan.
