That's not powdery mildew: that's mold, most likely aerial stem blight. It's an extremely common issue in greenhouses because the high moisture in the air, poor ventilation and mild temperatures are their ideal growing conditions.
How to deal with it? The best cure is to provide ventilation to your greenhouse and to lower air moisture. Yes, there are fungicides which are effective against molds but in all honesty I cannot recommend them unless you have tried everything to remove favorable growing conditions for the molds.
Also remember one thing: weather in London is not so cold as to require putting pomegranate plants in a greenhouse. I have a pomegranate planted in the garden and he just lives there with the snow and all the rest: it just drops its leaves in the Fall and restarts in the Spring. You'd be probably much better off putting them against the side of your house (South is better) and use an old bamboo mat to give them a modicum of protection.
While edible varieties do their best in the heat they can survive cold weather no problem and ornamental pomegranates (usually belonging to the dward/nana group) grow well in most of Europe.
