I ate at a Pizza Express a few days ago, while in a town (Kendal) with virtually no vegan-friendly places open in the evening.
I had olives and almonds to nibble on, plus the Veneziana pizza (Red onions, capers, olives, sultanas and pine kernels), with no cheese and with extra roast vegetables.
The service was fast and friendly, the pine kernels were generously sprinkled, and we didn't get charged for the extra topping. I was pleased with the food and would go back.
The size of your pizza depends on whether you go for the thick base or thin base. We opted for thin base and it was about the size of a 12 inch LP; bigger than the plate it came on, anyway.