How about this, then:
Can you conduct the flight under para 15? If you can then great. If you cannot, then you need 3 crew for 13 hours, or 4 crew for 18.
That's in any direction. (For all you know, we may be asked to operate LON-JNB one day - which AFTLs will you use then?)
How many time zones you are away when you finish your duty determines which rest applies - see Table R.
"ULR" is just a name, it doesn't change anything.
And that's it, really.