Neil,
I have the 650D and it is a cracking camera, im sure the results you will get from the 600D will be great.
The first thing I swapped was the standard 18-55 kit lens for the Tamron 17-50 f/2.8 VC, which is a relative bargain compared to Canon's 17-55 equivalent. The kit lens gave me about £70 towards the cost when I sold it.
I also own the 50mm f/1.8; I have used it maybe 3 times in total, I find the quality of the Tamron is that good that I dont need to use it.
I have a Canon 55-250 IS which only cost me about £110 brand new; this is a bargain and I only wanted a longer lens for rare occasions when Im watching motorsport.
With regards a flash, I have a Yongnuo 565 which is great; it works off camera without any additional wireless triggers. It is a TTL (Through The Lens), so it is totally automatic.
Get a decent bag for holding everything in, and also a smaller front loading one if you want to just take the camera and one lens with you.
Oh and if you havent already bought it from the link you posted, it is £20 cheaper in Currys with the same deal.
Hope this helps