Some bitters use invert sugar such as Lyle’s Golden Syrup or you can find info how to make invert sugar at home.
You can’t go wrong with Maris Otter or Golden Promise as a base malt. About 5% Crystal malt, as noted in previous posts. Or some recipes call for amber malt. Though its not traditional, I also like to use a little character malt such as Victory malt, light Munich, Biscuit or Aromatic malt. Maybe 2 or 3 %.
Traditional bittering hop choices are Northdown (one of my favorites), Challenger, Target. Kent Goldings is great to use for any or all additions. Fuggle is softer, best for late additions. I discovered First Gold and that is now about my favorite hop for these styles. The last couple I made were all First Gold.
Far as yeast, the 1275 comes off as too clean or neutral for me. I’ve made ribbon Mild Ales with it. Not so much bitter. To me, it’s almost the same as 1056 American Ale yeast. (I’m a Wyeast guy.) I have had great results with 1968. There is a 1768 Cask Ale but I think its a limited release and I have not used it. The other London yeast is 1028, which has too high of an alc tolerance for me for bitter and its better suited to stronger and darker ales like porter. People rave about the 1469, supposedly Timothy Taylor but not really. I’ve not had good luck with that one either. Maybe because I’m not fermenting in a Burton Union system. 1318 is sometimes reported as Boddington’s yeast. I’ve had ok results with that. White Labs has 2 or 3 strains I’d like to try like the WLP025, WLP026 but they are vault strains and the vault never seems to open. “Only 113 more orders til your yeast ships.”