As a consumer in the UK, how can I choose well marbled beef?
With online meat shopping in the UK you have to go with the reputation of the source, there's no grading system which you can refer to. Top end butchers and grocery stores carry top end beef, and most of the time you do get what you pay for. Many supermarket chains have premium labels, ie Tesco Finest and Sainsbury's Taste The Difference, which are definitely much better than their regular stuff, but I think the best mainstream grocery store premium brand for meat is Coop. 

At the end of the day there's no substitute for getting your own eyes on the meat you are buying, so I would recommend you try local butcher shops. There's still plenty of them around in the UK, and the quality varies dramatically. There are some that will pawn off legs of lamb that are about to turn, and others which control their entire supply chain and have some of the best meat in the world. You just have to try a few and see.