The EE tent, you queue to get in, and then sit and wait while your device charges. I don't believe you can charge a power bank in there but if you did it would take several hours. They will be doing power bank swaps, think you buy one for £15 and then you can swap it once per day for free. But I think these are only 5000mAh batteries, so will barely fully charge a modern phone.
In years gone by there was a guy in the craft field (or maybe Undle Ground), he had a van and a load of solar panels, and you could leave your phone/power bank with him and he would charge them for you. Think it was £5 for a phone and £10 for a power bank.