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I take your point that this particular service is near empty, however when you think about the bigger picture; you may find that other services in its diagram are not 'near empty' but however need the 8 cars.
Unsure if its related but Brighton - Victoria services are diverted via West Coastway today formed of 8 coaches (12 from Littlehampton), so it might go back to Brighton and then form a London service - hence why its 8 coaches.
Quite similar to Glasgow - Ayr weekend services
They start off as 6 cars, often very lightly loaded up to 08:00 (Saturday) and 11:00 (Sunday)
However later in the day and especially in the evening they are well loaded, often full
So rather than try and estimate passenger loads and the hassle of joining sets up during the day, they are just operated all day
Unsure if its related but Brighton - Victoria services are diverted via West Coastway today formed of 8 coaches (12 from Littlehampton), so it might go back to Brighton and then form a London service - hence why its 8 coaches.
Barnham to Bognor is also closed today, so there could be all sorts of reasons for that service running as an 8 car, especially early morning as the service starts up.
Well Preston Park to Three Bridges was closed, so the train would have gone, Brighton-Littlehampton-London. Its possible that the 8 coaches went back from Portsmouth to Littlehampton and join 4 from Brighton,
Or it was 8 coaches down from London to Littlehampton where passengers transfered to a service to Brighton, and then your train continued to Pompy. This would explain the longer length to accomodate more passengers on the west coastway.