Travel Bleg — Phones in London

Quick travel bleg: what’s the best option for a short-term mobile phone in England? Rental? Cheap prepaid phone? Something else? And what’s the best place to get one? At Heathrow while you’re waiting for your bags? In town? This would mostly be for actual phone calls. Data/email is secondary.

No guesses, please, if you don’t actually have any personal experience here. But if you’ve done this recently, any advice or recommendations would be appreciated. Thanks.