Agree with Chris. Assuming you’re on the river, moor up at Tesco, walk into Tesco, grabbing a trolley to carry the cans in. Walk through the petrol station - you look a bit of a plonker walking filling up a trolley but used to do it all the time when I had a petrol boat!
I think your next option is Hobbs in Henley but its 1.80 a litre or so, next downstream would be Bray and the Windsor marinas. Nothing upstream anymore. Loads of Jerry cans is the way to go unfortunately.
Another option, although not as close as those previously mentioned, is this one that I used last time I was there in a petrol boat. Moor at the small X, walk along Scours lane to the roundabout and fill at the Shell garage (bigger X). Not as far as it looks.
Tesco definitely the best one if it is a boat specially if coming out of T&K as it's about 5 minutes upstream and they have a proper mooring.
Not been there this year so can't comment on the current availability of the mooring to be fair. It might have been annexed again by wrongdoers.
ETA I can't unbold the text. Bold text was not intentional.