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Stormbird and Hermits in Camberwell. The real ale and the craft beer on opposite sides of the road.
If you wanna do the full real ale experience, The Hope in Carshalton is probably the best in London. At least CAMRA think so. Depending on when your mate is arriving there’s often a beer festival on there.
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The Harp in Covent Garden is the obvious. Southampton Arms is very good if you go a bit further north and also near Tapping the Admiral and the Pineapple.
The Market Porter and the George around London Bridge are both good historic pubs and nearby the Bunch of Grapes and Old KIng's Head are more traditional boozers.
Where's good for real ale in a nice, traditional pub? I've got a mate visiting from Slovakia in a couple of weeks who loves all that stuff. Probably looking at South/Central/East.