I agree with this and it is quite common for British people to have mixed heritage- Scottish, Welsh and non British heritage of Irish. I just think it is confirmation bias these interactions on holiday prove your views i.e I am more likeable than the English.
As I posted above, English is so commonly spoken across the world that in these small interactions on holiday e.g a shopkeeper serving you, they are obviously making assumptions based on your looks combined with the English speaking so you may correct them with the, "I am Scottish, I am Irish etc " but I think you are treating their response as more salient than it is as it aligns with the views you hold personally. Despite being English, speaking English, I am regularly assumed to be from a Scandinavian country, in the past the Spanish and Italian waiters didn't think I was English, they just assumed that I was using English to communicate with them as more commonly spoken across the world and they thought I was Swedish and Danish based on their own assumptions on what that looks like. I have Scandinavian heritage so not that shocking but when I corrected them, they didn't suddenly change the way they spoke to me, just as friendly, just as chatty. When I was in Denmark, I was always spoken to in Danish, when I made it known that I was English, they were still pleasant and polite.
As for colonialism, have a read of Heart of Darkness, (Conrad) if you think only the English were horrific colonialists, the Belgium's were pretty bad, as were the French as were the Germans(Prussia) as were the Spanish as were the Portuguese and of course the British which includes all of Britain!!