I could probably afford J more often but I begrudge paying say £2000 for a LHR-JFK ticket when I can fly ex-EU for less than half the price sometimes or fly in Y for a 5th of the cost.

I build up Avios over the year, use these, book things when sales are on etc.
I'll go somewhere that's cheap as well i.e if DUB-HNL is £800 and DUB-JFK is £900, I'm off to Hawaii. If I need to go to New York and there's nothing cheap in J but I can pick up Y for £300, economy it is. These earn the Avios that allow me to fly J/F on various flights and in the long run, I've more in my pocket for other things.