Ah, my old stomping grounds...
Yes, you can get the same price ticket from Bognor or Littlehampton.
I've never sure of the validity involving Littlehampton on a Bognor ticket but I'm pretty sure it's OK.
Perhaps more usefully though is that the Bognor ticket opens up the option of the routes via Horsham should you ever want / need to go that way.
For no extra money I can't see why you wouldn't go this.
Cheers,
Mr Toad
EDIT:
If you limit yourself to "Southern Services Only" then the ticket from Portsmouth Harbour to London Terminals is actually £100 *cheaper* then the ticket from Worthing. However that does limit you to Southern trains only and I have no idea how they are interpreting Thameslink trains on the Brighton line these days.
Another option:
The annual Worthing to London ticket is £4,560.00.
What about a (Any Permitted) Havant to London ticket for £4,676.00? Yes it's a little more but it also gives you the ability to use Worthing - Brighton - London, Worthing - Horsham - London and Worthing - Havant - Guildford - London and any points in-between.
I've no idea if that's worthwhile for you but it always is worth looking at stations further away when renewing season tickets.