I have been a member of the Budokwai for years and aside from being the oldest club in the UK it is a very friendly place. I only do judo but have watched other martial arts being taught and the standard of tuition seems very high.
You pay an annual membership and get a reduced fee for your training sessions or you pay as a visitor. Judo is very reasonably priced and they offer absolute beginner courses with a judo suit included...bargain! The kids classes are always busy which I assume means they are equally good. Recommended.
Budokwai Judo, Karate & Aikido is the oldest international Judo club in Europe; all standards and all ages are welcome. There is also a Budokwai toddlers play group.