One thing you don't really need right now - the flash gun!
The built in flash should be sufficient for what you are doing now. Once you know a more powerful and importantly a more flexible flashgun is needed,then get one and invest in a half decent one at that. Don't waste money on something you might not use.
Moreover,if you like the D5100,I would get it. Think how long you are going to keep the system. I don't see why a D5100 should be not enough for at least the next three years,so think a bit longer term. All up its around £100 to £150 more than the cheaper cameras like the D3100 and Pentax K-X dual lens kits. Not a small amount of money,but over a few years not so bad.
Also,while you are down at Jessop,I suggest you look at the Sony A37 which shares a similar 16MP sensor as the D5100 and uses the A mount(will work with decades of Minolta AF lenses). Might be worth a gander at too and is a bit cheaper AFAIK than the Nikon D5100.