Your London Terminals ticket (I’ve got one too) allows you roughly speaking to travel to any of the mainline London stations on your side of London. I once took the piss using mine to go from South London to St Pancras and had my ticket scribbled on by an officious gate guard. So you can hop on a tube from London Bridge to Waterloo on that ticket fair enough.
You can’t get a point to point ticket between London Underground stations, but if you’re at a zone 1 NR station and you want to go to another zone 1 station then the fare you owe is the zone 1 single fare; it’s as simple as that. That’s the reason why your point to point ticket is cheaper than a Travelcard: it’s because it isn’t a Travelcard.