You need to look at it from this way:
Your system looks good and will hold you over for quite some time but if you get a 6850 now you will not be able to enjoy games at the highest setting for long. In fact that card will have problems with current games.
And then there's new money to be spent.
I think the absolute best value for money is the 6870, it is about ~20 pound more than the 6850 but I really think you are getting your money's worth.
If you cant afford it now but can wait a little longer I would advise to wait another month or two, which is also what i am doing. GTX 560 Ti's might also be available for a lot less by then if you are able to spend more, otherwise 6870 will still be a great choice.