Okay I know canonically Allen is an Englishman, but hear me out! I actually believe that Allen is Scottish!
So first there is his name: Allen, it was historical a more common name in Scotland than in England. Second there is the fact that Mana Walker (his adoptive father) was raised as a Campbell in his early life, and Campbell is definitely a Scottish surname. So that means that Mana spoke with a Scottish accent and since Allen speaks exactly like Mana then that means that Allen speaks in an Scottish accent as well. (I’m in love with this idea btw) There is also the fact that Walker is a common Scottish Surname as well! Plus Allen’s clown outfit looks like it was inspired by kilts.
Headcanon: Allen Walker is Scottish! and he looks boss in a kilt!