Well,I don't have one that I keep around in the usual sense,But I do use Cooper's OS Lager & OS draught as a bittered base for several other styles I like. And I'll def make more of the coopers International Series English bitter,with tweaks of course.
I just add different plain malts & hops to change it to what I want vs what you'd normally get from them. That's part of the fun to me. By this time next year,I should have a short list of ales I'll be keeping in stock.