The largest ever locomotive used in Britain was Gresley's solitary U1, a 2-8-0+0-8-2 Garrett used as a banking locomotive on the Worsborough Incline between Barnsley and Penistone, both in Yorkshire. See the Wikipedia article

Next in size were another class of thirty-three LMS Garrett

Next down from that were an experimental 4-6-4 by Gresley, and the unsuccessful (or rather, uneconomic) 2-8-2 P1 class, also by Gresley. After that were probably the very successful LMS Duchesses, and then lots of other 4-6-2 Pacific classes.

British steam designs were always limited by loading gauge and axle weight restrictions. The Duchesses, the Gresley A4 class and the BR 9F 2-10-0 heavy freight locomotives probably represented the ultimate possible power from traditional designs based on one boiler mounted above the running gear.