Wouldn’t you be benefiting from your friends? It’s ok for a little bit, but if they live there permanently then they will pay off your mortgage and have nothing to show for it themselves. That sort of thing might build resentment long term. Though in the short term you both benefit.
But as I’m sure you’re aware, any money issues may sour the relationship. Even just having a formal contract with exchange of money could change the dynamic drastically.
UK kettles use the standard 240V mains and it’s excellent. Under 2 minutes to boil 3 US Pints with a 3000 Watt kettle.