We, not usual for us, filled up in Frome at ASDA on Saturday before travelling to Portsmouth for our ferry. 150.7p per litre and a 3% Santander cashback that brings it down to 146.18p.
Most of the stations we passed on route in France were north of 1.83€, but came across an unmanned one in a village called Saint-Branchs south of Tours at 1.74€ (£1.499, £1.484 with Chase 1% cashback).
So at the moment fill up in UK the cheapest you can find, nearest your leaving port.