If you're looking at One World airlines and only want one stop that's a limited selection.... Nothing popping up on Finnair via HEL. I do know Iberia do a direct flight from Madrid to Guatemala so you could take a look at that.
Outside of one world if you wanted to avoid US transit then Aeromexico fly a 787 LHR to Mexico City then its a brief hop to Guatemala, from memory the Heathrow to Mexico City is overnight as well so you're not burning a day