Key items that earn their weight: a lightweight microfibre towel (many accommodation options charge extra for towels), a small padlock for hostel lockers, a universal power adapter (essential in the UK and Ireland), noise-cancelling earbuds for overnight trains, and a day bag — a small packable tote that folds into your main bag.
What to leave behind: hair dryers (use the one in the accommodation), more than two pairs of shoes, full-size toiletries (buy on arrival), guidebooks (your phone does this), and anything you're bringing 'just in case'. If you haven't used it in the last two trips, leave it.