I don't remember the behavior of the pumps being different in England, but I only drove there a couple of weeks, compared to 30+ years in the US, so consider this to be USA-based advice. (I suspect the auto cut-off on the pumps is essentially the same hardware in both countries, too.)

I've never found the tiny difference that load makes to make any difference in filling the tank. There's more difference than that in slope of pavement in filling stations. I am always careful to put the nozzle "straight" in to the filler opening, which may remove some variables.

The shutoff is back-pressure based, so if something is restricting fuel flowing into your tank somehow, that could cause this symptom.