I use the White Company and Cologne and Cotton - John Lewis does give great advice on thread count and I do use them too
I actually found really nice ones at H&M.
For example: 0821679001
I hope the code is correctā¦ Iām from Germany and I feel like sometimes we have different codes 
I have the Fired Earth x M&S bedding. Itās a washed cotton with thin flange trim so has a super relaxed look. They have a really nice palette based on Fired Earth paints, I went for Dover Cliffs which is a chic off white rather than crisp stark white. Love!
I donāt think you can go wrong with Dorma bedding. Iād say its mid price range but excellent quality. Dunhelm stock some of their range but bettter to just go straight to their website I find.
Cologne and Cotton do really lovely bedding too! x
I would 100% recommend Tielle. They supply bedding to luxury hotels like The Connaught, The Pig hotels, The Savoy, Claridgeās, The Newt, Soho Houses etc. I have slowly been changing all my bedding away from The White Company to Tielle as it lasts so well (no shrinking/bobbling).
Iāve seen a few people recommend Dusk but I returned the bed set I recently ordered from them as the quality was so poor.