You are entitled to one full journey on LUL on the outward and return
legs. The only way you could "break" a journey is at an out of station
interchange where the gates are configured to allow such a break but
with a time limit in place.

Out of interest, do you know how long that time limit is?
I presume the same time limit applies for Oyster Pre-Pay in similar
out-of-staion interchange situations?
I've also wondered if Oyster Pre-Pay allows for such out-of-station
interchanges at more locations than paper ticketing would.

Aldgate Fenchurch Street becomes a single journey on Oyster but
Bank Fenchurch Street does not, which is odd as I seem to remember
Bank being a valid station for tube X-London transfer.

The time limit has been long enough for me to sit and have lunch at
Paddington between changing from H&C to Circle and still count as one
journey. It must be at least an hour.