The little furry white things ARE some sort of creature, they are either wooly aphids or something else nasty. Wipe them off as best you can. Serissas are one of the most difficult trees to keep alive, and we rave here all the time about selling serissas to beginners because they cause heartache and make people give up on bonsai. It's like giving up driving because you can't handle a race car your first time out.
First, serissas need lots of light when grown indoors. You may have to rig a fluorescent light above the tree to give it enough light to grow. It's too cold in England for the tree to go outside right now, that's probably true.
It's also usually too dry inside of a house. Trees need humidity. Especially if it is near a heat source. You can help this a little bit by filling that tray you have it in, with a coarse sand, and then sitting the serissa on top. Don't let the serissa sit in the water, but keep the sand wet. You could even raise the tree slightly above the sand with little pebbles or something. The idea is to keep moisture IN THE AIR right around the tree.
Finally, don't water too much, and stop feeding it. If the roots are unhealthy they will get harmed further by the fertilizers. Wait until you see new growth for a while, before feeding it again.
Make sure that the drain holes on the bottom of the pot are not covered by something. Water should run out freely when you do water. Which will only be when it needs it.
Good luck, and let us know if you need further information (like about rigging the lighting if you decide to do that) and next time we'll be glad to help you choose a tree that will live better for you! There are also good bonsai nurseries in England, as well as clubs to join. Clubs are great for beginners (and anyone) because they help you with local growing information..
Oh, and I'm a beginner too, so all of this information is garnered from reading and paying attention. So if anyone has better advice, and more experience, perhaps you will get another perspective.
Joanie