While it may be and "odd" route, why were you on the flights, do you include yourself as "older folks" from England, if not then other people could be using the route for the same reason as you do.
I doubt it. I took the flight because a couple of months ago JP had some sort of semi error fare when they had one of their rare promotions and I could book MAN-LJU-BRU with a stopover in LJU for 45 GBP all in.
Slovenia is indeed well worth a visit. Another option to get to Slovenia I have used in the past is going to Klagenfurt and taking the train from there - although KLU is in a rather sad state schedule wise