Ok why does Americans posting snotty things about UK food make me so furious? I'm pretty sure it's not because I'm a secret nationalist

It's reflexive nationalism. Tribalism and ingroups are a part of everyone's identity even if we don't make it a big part of our beliefs. So even if you're not really committed to the ingroup of Britain it's still annoys you that an outgroup is ignorantly attacking something familiar to you.