According to Elderly Lister's tattoo back in the second episode, they probably didn't. (Either that, or this was part of an Aborted Arc that was scrapped when the show got a better budget and could do cooler things.)
There is no sign of Elderly Lister having a tattoo in the second episode. Certainly not one that says "U = BTL", which would suggest far more forward planning that Grant and Naylor have ever suggested was the case. That only happens in the book.
I pulled this from the Red Dwarf entry:
There is no sign of Elderly Lister having a tattoo in the second episode. Certainly not one that says "U = BTL", which would suggest far more forward planning that Grant and Naylor have ever suggested was the case. That only happens in the book.