Bearing in mind that the "any permitted" is cheaper than the "via Manchester" fare it would be worthwhile checking this against the routing guide as I suspect it does not include travelling via Manchester.
Clearly the enquirer needs to check this carefully. However, even if he opts for the cheaper ticket I imagine the option exists to "excess" the ticket for the return journey via a more expensive (but still valid) route. The trick is to find a ticket office that will fully understand the process!
I was foolish enough to take the bait
The National Rail journey planner didn't seem to want to provide the "any permitted" fare for a journey via Manchester, so I checked up the valid routes using the online Routeing Guide. The new "Routeing Point Calculator" allows one to find the valid Routeing Point for Slaithwaite, and the only one valid is Huddersfield (HUD). Stalybridge (SYB) is not a valid Routeing Point because the London-SYB fare is higher than the London-
SWT▸ fare. Stalybridge not being a valid routeing point rules out going via Manchester.
But read on...
Looking at the Route Maps for valid routes from London to the HUD Routeing Point, one finds that Manchester/Stalybridge is a valid route. The only reason I can see why London-Manchester-Slaithwaite isn't permitted is because going that route to SWT means doubling-back from HUD, which is not allowed.
So, you can buy an "any permitted" London-Terminals to Huddersfield ticket, and it's valid via Manchester.
And (for fares where there are no specific exclusions), you can break your journey at Slaithwaite.
Just one snag
. If you end up going via Kings Cross and the East Coast, your Huddersfield ticket will not get you to Slaithwaite. You will need to get a distance excess...and as the London-Slaithwaite fare is (for most walk-up types) the same as the Huddersfield fare, the excess should be zero.
That is my analysis, without guarantee of correctness. I would be grateful if other forum members could check this; particularly the ability to ask for the zero-fare excess.