Fullers ESB is one of my all time favorite beers, dating back to when it first appeared in the USA. About the best you can do if you are attempting to duplicate it is to use Windsor or S-33 in my opinion. That's what I've done over the years, mostly with Windsor. And it doesn't work all that well as a good substitute to be honest, or at least it hasn't for me. If I want it to finish at a lower final gravity I toss in 1 pack each of Windsor (or S-33) and one pack of S-O4.
S-33 and Windsor are pretty close overall.