Oh, you can make tea from borage? I have the loveliest, healthiest borage plant. I use it in my salads and I like the flowers for decoration in an ice cube in summer drinks.
My (huge-out of control) plant has an intense flavor, stronger than I remember from my garden in Germany.
I was going to suggest lemon balm and orange mint as a nice combo. Lemon verbena is totally different and delicious, (awesome with fish) but you may not have the right climate for it in the UK. I live in Florida and it is definitely a sun lover.
Chocolate mint is one of my faves for both cooking (with carrots for example) and tea - divine flavor.
I also have a couple of deliciously sweet spearmint plants and some strong, fresh peppermint, a refreshing treat in the heat.
The leaves of my pineapple sage smell divine, but I am not certain if they are edible. I'll need to read up on that.
There is a variety of hibiscus that makes a lovely, fruity cup of tea - it's growing wild in my backyard, so I am ready to give that a try soon. Can't grow the German chamomile here, fizzes out in the heat, but it is the best and most healing kind for tea. UK should be the perfect climate.
The one mint I haven't tried yet is a Puerto Rican bush mint. It took a while to acclimate. It is a mini shrub and the leaves have a mild, sweet mint flavor - I thought I'd mix it up with the lemon balm - should balance out nicely.
Oh, and a native wild violet with pretty white and lavender blue flowers - the knowledgeable local plant lady said it has been tested and used in treatment of cancer and intestinal issues by the medical center (University Studies and Clinic). It is now a thriving plant, so I figured it would be safe to harvest enough leaves for a cup of tea soon.
I rarely dry mine - mostly I use the leaves fresh from the garden in salads, smoothies and for tea.