Congratulations on getting a seed from a bonsai kit to sprout! I've never heard of one that was viable haha. I would agree with Auk that it looks like a Chinese Elm, in which case you have a seedling that may eventually grow into a nice little tree. Once it has grown a little stronger, probably the best thing would be to plant it in your garden and just let it grow like crazy for a few years. I have an American elm that I have been allowing to grow to significant height (around 10 feet) before chopping it back to grow again the next year. Each time I will chop it back a little sooner so that the trunk tapers from thick at the base to relatively thin at the top. I've no doubt that at the end of the process I will have an ugly but strong tree that hopefully I can then develop into a bonsai. I would recommend you attempt something similar with your tree, at least for a year or two to thicken the trunk somewhat. In the meantime I believe that the UK has a thriving bonsai community, and you should be able to find a club to learn more about the art. Also, you can check out mikbonsai (a uk bonsaist) on youtube which you may find instructive and interesting.