In the UK, oak, larch, field maple, japanese maple, hawthorn are good species.
I woudl hoever to not start those from seed, but rather start with some larger plants. Growing from seed takes a very long time before you can actually do much besides watering. And at that point you still have no experience and do not know what to do. By al means, do also grow from seed but also start some projects by purchasing starter material. Kaizen bonsai in de UK is a good place to get something. Or on one of the shows in the country.