One of the most exciting gardening hubs to be developed in recent years is surely The Eden Project in Cornwall. If you don't know the Eden Project - it's a disused clay mine, which has been transformed into a rich, global garden where people can learn about nature and get inspiration about gardening and horticulture.
Eden is not only a great day out in Cornwall, an educational charity and social enterprise which has a strong focus on sustainability. In terms of gardening classes, you can do the Royal Botanic Garden Edinburgh/Eden Project Certificate in Practical Horticulture at the Eden Centre.
If you're interested in more specific gardening courses however, you can find horticultural subjects from experts - brought directly to your own home at MyGardenSchool.