The 025 (Worthing) - Brighton - London Victoria used to go by a much more direct route. Presumably National Express find more people find its route via Heathrow to be useful than did the speedier journey.Thanks for the comments everyone.
I guess I'm a bit surprised as I always imagined airports charged coaches (and everyone else) heavily to let passengers on and off at their airport (on the basis that it's the airport which has the monopoly on access to the location) and coaches naturally had to pass that on. And it seemed a bit much that they charged that for passengers who were just changing coach there. However, it looks like it is is the coach and bus companies taking the 'excess' profit.
After a bit more poking around the web I thought a possible reason for there being no direct service from LGW to Victoria (not going via Heathrow) is that there is good train service.
In fact LGW-Victoria rail station and Victoria coach station to wherever seems cheaper - and sometimes quicker - even if using a Coachcard! Provided you're willing to walk between the two Victorias which I certainly would without luggage. Or indeed with luggage, but my wife doesn't feel quite the same way about that.
I used to use that coach from time to time, but can't imagine saving enough on it now to be worth the extra journey time.