For gaming it is unlikely you are ever coming even remotely close to 8GB. In that case you would see no performance increase from 16GB and would be better off with the 8GB of 1866 Mhz RAM. That being said the performance increase from 1600 Mhz to 1866 Mhz in most games is insubstantial and I would consider the money better spent toward the GPU.
If you are using the computer for video editing or another application that demands a ton of memory I would go for the 16GB to remove that bottleneck.
I would put all £90 toward the GPU. The RAM you have currently is fine. Look at the HD7770 and see if you can catch a good deal. I'm not sure on the exact change rate to Eur but it should be right around that price range.
Edit: If not, the 7750 will still be miles ahead of the 6670.