Sheerness On Sea has associated routeing points: Chatham and Faversham.
Tamworth is a routeing point.
Fares Check
Sheerness On Sea - Tamworth
Anytime Single
Route +Any Permitted
£97.50
Chatham - Tamworth
Anytime Single
Route +Any Permitted
£95.00
Faversham - Tamworth
Anytime Single
Route +Any Permitted
£97.00
So Chatham and Faversham are both appropriate routeing points.
Determine Map Permissions
Your permitted route must be the shortest route (or those no more than 3 miles longer) with a regular passenger service to either Chatham or Faversham, and then along a mappped route between Chatham or Faversham and Tamworth.
Chatham - Tamworth: London
Faversham - Tamworth: London, CS+LR, TL+CS+LR, TV+LR, WM+TV+LR
Using 'London' routeing, we also have the following:
Chatham - London Group: HK, HS, LD, LR, NK, SL+LR, VC
Faversham - London Group: HK, LD, LR, NK, SL+LR, VC, VC+DV
London Groups - Tamworth: TV, TV+WM
Any route traceable using the map combinations above for Faversham - Tamworth, or Chatham - London Group plus London Group - Tamworth, or Faversham - London Group plus London Group - Tamworth, without doubling back (except within the same Station Group for interchange purposes), is a permitted route. This includes HS1, if so desired, and transfer on the underground between appropriate London Terminals.
If you get stuck then let us know.