As title. If I buy a flexible ticket from a station where the route to Brighton / Hove is via Preston Park, can it be used at both Brighton / Hove no matter the ticketed destination is Brighton or Hove?
Hove is a member of the Brighton Routeing Point Group; from the north, going via Hove is also less than 3 miles longer than the shortest route to Brighton.
Therefore in almost all circumstances it will be permisible to use a Brighton ticket at Hove, and vice versa.
I suppose there could be an edge case scenario where you can't go via Preston Park-Hove-Brighton, as going via Preston Park is
only just within 3 miles of the shortest route to begin with. For instance, for Haslemere to Brighton, the shortest route is Barnham-Hove-Brighton at 61.05 miles, whilst going Redhill-Preston Park-Brighton is 63.12 miles. Going Redhill-Preston Park-Hove-Brighton would be 64.55 miles, so more than 3 miles longer.
But in almost all such examples, going via Hove would be a mapped route anyway, so the fact that it's more than 3 miles longer than the shortest route doesn't matter.
The only other circumstance in which it might not be permitted is if you're relying on the 'direct trains rule' to validate your route, as by introducing an additional change it ceases to be a direct train.