I was looking in M&S for a suit last year, and told an assistant that although the 30" trousers were an OK fit at the time, I could easily drop 2" from my waist as I was a good ten pounds overweight, and I'd be best off buying a 28". She looked a bit shocked and said they were boys' trousers
Funnily enough I was back there last week trying on jackets again, and my old size of 36" regular is now too long in the sleeve and the back, and I'm now better off in a 36" short. So their patterns are changing over the years - chests are getting smaller vs height and waists are getting larger. It can even be worth having a root around in Primark: sme of their trousers and jackets in wool blends are not bad value (and are usually available in 28" waist), and appear to be the same quality i.e., using identical fabrics, linings and cuts as value online 'traditional tailors' like Brook Taverner, but at half their permanently discounted prices. I used get a lot of stuff in size S from Uniqlo, but I rarely bother these days as the quality has taken a nosedive in the last 3-4 years. And good luck with finding anything smaller than a 30" in TK Maxx!
To add to Biggsy's suggestion, look for a waistband that is split at the back as it's a lot easier to adjust. (Mum used to teach fashion and dressmaking, so I picked a lot of this stuff up. I even had a go at making a shirt once from a pattern)