To the best of my knowledge, one reason why the 442s were transferred to Gatwick Express was because SWT decided to reintroduce all of the 458s back into service (before February 2007 only five or six 458s were in revenue operating serving, and prior to this SWT considered handing back the entire fleet to Porterbrook due to their atrocious reliability). This would have meant enough 450s could have been displaced from the Windsor line services to work on the Portsmouth Direct Line, allowing for the transfer of 444s to the Weymouth line to allow the 442s to be withdrawn.
Correct me if I'm wrong, but I recall reading somewhere that the rationale for this decision was because the leasing costs per mile for 458s were considerably lower than for 442s. This would explain why the 442 refurbishment program, underway by February 2007, was cancelled as soon as Stagecoach won the South Western franchise re-letting.
Nonetheless, it was a stupid decision in terms of passenger comfort. I find the seats on 442s considerably more comfortable than the Grammer seats on 444s, and 442s were more suitable for journeys to Bournemouth due to their greater luggage capacity. A better solution would have been to order 24 extra 377s for Southern allowing for the disbanding of the 460 fleet, assuming the then SRA would have allowed it.