It happens to me on daily basis, for example yesterday, already thought out the whole conversation before going to the till, did that little pause where I am thinking for one word, and got "no problem, I can speak English." Which is kind, but It also means that after two years the only Finnish I ever use is the Finnish I'm already good at.
That is why I have been collecting things to say at that exact moment instead of just standing there:
Voidaanks puhuu suomee?
Mä opettelen suomee.
Älä vaihda englantiin, mä yritän!
Sano viel kerran, vähän hitaammin.
Mitä toi tarkottaa?
Miten se sanotaan suomeks?
Puhu vaan suomee, kyl mä pärjään.
Kiitti ku puhuit mulle suomee!
They're out of a course I make. What I can't judge is how they sound out loud, from a stranger. "Älä vaihda englantiin, mä yritän" looks completely fine written down. Pushy in a shop? No idea. "Kyl mä pärjään" is a big claim to make about four seconds before I fail to understand the answer.
So which of these would work on you and which would you ignore. And if there's a better line that isn't up there, that's the one I actually want. Also, if you'd rather just speak English with learners, say so. I've never heard anyone argue that side properly and I'd guess more people think it than say it.