You will have to pay each operator separately and each operator has different rues and amounts charged. South West Trains is £5. However you usually have to have an off-peak ticket. The upgrade is not generally available for Advanced tickets. This explains which operators offer Weekend First http://www.nationalrail.co.uk/times_fares/pr3744bcc35ce7410024862c7ece4649.aspx
Thank you Cathy. I travelled yesterday and was able to upgrade on my journey to London for £10. The rest of my journey wasn't busy so I didn't upgrade.
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