It's a problem with many Better gyms. Demand for classes often exceeds capacity.

Also, Forest Hill has a lighter programme than some other Better sites due to limitations on the dry side capacity.

If you are really keen, you could explore some of the membership options which would allow you to use other gyms besides Forest Hill, either just the ones in Lewisham or more widely such as Crystal Palace.

I'm a confirmed gym bunny myself, so I'm often to be seen at 10:00pm logging into a class for the following week. I also use more than one gym as there are a couple of coaches whose classes I've been attending for years, and these coaches are fairly peripatetic.