Baby sensory has a sister class called 'toddler sense' for 13months plus. This is a nationwide chain. You may also find some bigger kids classes start around 12 months (or 18months if not) e.g. football, ballet, playball, rugby, gymnastics. However, I found with my first, that actually at this age, she just wanted to explore at her own pace, rather than a structured class, so we went to soft play / gymnastics free play sessions, where she could do what she wanted, and whilst I had to watch her, it's a lot more child friendly/safe than my house!
We go to a class at the local library called rattle rhyme and roll. There are lots of different ages from a few months up 3 years old. At the local Children’s centre we’re moving from the under 1s stay and play to Under 5s! Seems like a big leap to me. I also go to Singing mamas where the babies range hugely in age. That one is for the mums mainly but most have young children in tow
My baby enjoys Hartbeeps, I think they have them across the country. They have 3 classes separated by age so it’s at the right level for him.