There are no day return fares from Swindon to Southampton Airport - it's considered too long a journey. However, if you are looking to do the round trip in a day, a day return Swindon to Warminster and a day return Warminster to Southampton Airport add up to £37.70; all of the ticket prices quoted are any time, but off peak fares at lower rates are available on all of them if you're staring your journey after the morning peak, or travelling at the weekend.
An Off Peak Day Return (
CDR▸ ) from Swindon to Southampton Airport Parkway/Southampton Central.
Swindon - Warminster CDR £7.40
Warminster - Salisbury CDR £7.40
Salisbury - Southampton Airport Parkway/Southampton Central CDR £10.50
Total £25.30. Or,
Swindon - Didcot Parkway CDR £11.20
Didcot Parkway - Basingstoke CDR £9.50
Basingstoke - Southampton Airport Parkway/Southampton Central CDR £15.00
Total £35.70.