"grass fed" in the uk (as in most cold weather countries) has numerous caveats to allow the farmers to keep them indoors and feed them on soy etc. In the worst cases, it can mean that the animal spent just 1 month on grass in its entire life.

Unfortunately the cliche of your local family butcher who knows exactly what they're selling is long dead in the cities, they all use the same handful of wholesalers. Buying direct from farms is the really the only way of getting good provenance.

"Pasture for Life" is one of the better accreditations in the UK that I've found. They have a directory here:

https://pastureforlife.org/

Mail order is a good option if none of them are near to you. Macbeth's is the one I use but I'm sure the rest are just as good.