After school swimming lessons

We use GL1. They run lessons after school and on weekends.
Reception are usually fairly helpful and can tell you what times certain levels are available, even if they don't have spaces on that one.
They manage things using an online system so you can see the progress your child is making against each of the criteria. Once they pass a level they either move them automatically to the next level or the system shows you the classes available and you select and move them online. You can get them on a waiting list for other classes at that level. You can also stay in their existing class to wait for a convenient one to come up. Assessment is continuous.

Cheltenham have after school lessons at Leisure@Cheltenham. I don't know if they've changed it now, but classes used to have to be booked in person each term. Booking opened at something like 9am on a weekday. If you work or had to do the school run you could request by email but would be allocated after the in person bookings. I heard tales of people arriving hours before hand to be near the front of the queue. Lots of people switched to lessons elsewhere.

I also know of people who used an independent company at Dean Close at the weekends and heard good things.