I agree it is a narrow "apply a simple rule blindly, and assume our staff do not have capacity to make a valued judgement mentality" that most of the supermarkets and main brand pfs use....quoting "local authority" licensing rules.
They will not distinguish between a "fuel can" and an approved tank. I have the pump shut off after putting only 5l into a 25l external tank before now, at a supermarket pfs right on the coast.
I find the smaller independents are more understanding, and usually buy fuel from the one that sold me the 20 litre red plastic jerrycans.
I see the under the new regs:
What size containers can I use?
"The new Regulations state that petrol can be stored in the following
containers:
plastic containers up to 10 litres (previous regulations restricted this to
5 litres)
metal containers up to 20 litres (previous regulations restricted this to
10 litres
So there's a 10l plastic can available for petrol?
And
Up until now a 20l metal jerrycan was not a "suitable container" - surely not

I'm confused.
Might be worth pointing out what the regs actually say when next filling up at a supermarket pfs and see their reaction!