Hey! I'm from the UK (pending) too! Honestly the best thing I've found is to just let go of your pride. You're gonna stammer either way. Get angry, get red in the face. For me, it flows once I've laughed about it. Nearly once a day at work I'll say 'wow I can't speak today'. Nobody gives a shit. Second good tip is to act. Just act that you're slightly angry. Act like you're kinda upset. Distract your brain. Act like your accent is slightly different. Just some tips that have helped me with phone calls and stuff.