For eligible age groups. At least, it looks like that's what happening.
I had my first Pfizer as a walk in with spare doses (pregnant), and have tried to book on the website just now for my second jab - it's only showing vaccination sites that are 30+ miles away, which I assume are the nearest hubs with Pfizer stock. The times are also highly limited (you can only book a week or so in advance), which probably accounts for the unpredictability of stock.