Highly personal opinion in this post, but I'm not fond of "faking" so-called British English. Not only does it often turn into a sort of blanket stereotype, but imagine being British and reading sum feeked acc'nt when you literally have a common language, but with distinct spelling, vocabulary, and grammar. I'd suggest using that instead of othering the British readers by making them talk a bastardized version of their own language.
It's also weird to read a "British accent" when you're not from author's country of origin in general (let's assume US for now). I mean, think of it. I first of all need to assume everyone is speaking American English by default and then paste the accent on top of that default.
The exception, I think, is if your character has an accent that is distinct enough that a native Brit would write it differently - think Hagrid (apparently it's West Country English).
Basically, what I'm trying to say is that an accent is relative. It's default to the speaker, and only an accent to a foreigner. When you modify sumthin' it's making an assumption about the reader's default language. Look at it from both sides instead!
Just my take on keeping it international.
(I'm sorry if this comes of as a bit negative, that's not my intention.)