Wait, so you plan on going through Eastern Europe/China, are you planning on travelling overland from Europe to SE Asia? If so I suspect that £1500 may not be enough, depending on how long you are going for that is. How long are you going for and what sort of activities do you plan on doing while you are there?
Instead of doing that, it could be a better idea to book a return flight to Bangkok/KL/Singapore now, and use the money you would have spent in Europe going to the likes of the South pacific islands, or even places such as Borneo (or other parts of Indonesia, perhaps the less explored eastern islands), the Philippines or Hong Kong, as you can get fairly cheap return flights around SE Asia, particularly if you book them well in advance, but last minute works well over there also.
Regardless, it sounds good. I wish there was something like this when I went on my gap year.
btw, I think that quite a lot of parts of SE Asia will be in the monsoon season during September/October, so if you don't like heavy showers everyday then it might be a good idea to check which regions will be drier at that time.