A plan is starting to come together now you have made me have another look at chargers near Soton.
I have spotted a Geniepoint, (Bonnet) not far from where we will be staying first night so that might be an option for a quick top up ready for the way back. Also there are Shell chargers (Bonnet) just off the M3 at Chandlers Ford and at Winchester.
Banbury at about 150 miles from starting point is easily do-able and may allow the option to tank it a bit (70mph instead of 65), then a splash at any of the ones I mentioned near Soton and then tank it back to Banbury......
Or is there a better plan lurking somewhere