We used a company called Siren training, they can come in or they also offer drop in classes around London resulting in certification of First aid competency. It last three years. they have a good quality/price ratio. I used them again when booking for volunteers in my workshop in Camden just earlier this year. Fully recommend them for all techs in the space.
There is a defibrilator in the Herne Hill train station, that said it would take at least 15 minutes to get it down to the arches so am in full support of making it happen for the arches. The price for one is between 500 - 1200 gbp.