Outdoors, I would start with a Common Juniper (Juniperus communis), or a Chinese Elm (Ulmus parvifolia 'Catlin'). Indoors, Willowleaf Ficus (Ficus neriifolia), dwarf Chinese Banyan (Ficus benjamina 'Wiandi', or 'Too Little'), or dwarf Hawaiian Umbrella Tree (Schefflera arboricola 'Luseanne'). If you have a spot with plenty of light indoors, a dwarf Jade Plant (Crassula ovata 'Crosby's Dwarf', or 'Hobbit') will practically do itself. Residents of the UK will probably have other suggestions--probably better than mine, since I don't live there! :)

You might want to get in touch with the UK National Bonsai Society, they will be able t ogive you lot of help and guidance especially if there is a local group in your area:

http://www.thenationalbonsaisociety.co.uk/index.html

http://www.bonsaimary.com/Beginner-Bonsai-Trees.html has good info on bonsai if you cannot source a local group. My sister used a self sown seedling cotoneaster from my garden and it worked well.

All above is great - thanks a lot