Anywaste, I am not very familiar with stores in the UK so based on what's available at www.dabs.com the best cards for £100 is either the AMD Radeon HD 7750 or the nVidia GeForce GTX 550Ti
You can get the following VTX3D Radeon HD 7750 for £79.99. Not sure how reliable VTX3D is; in fact I never heard of them. The card is probably overclocked to 1GHz, but the specs states 800MHz so there's some confusion there.
In terms of performance, both cards are evenly matched overall. However, the Radeon HD 7750 uses much less electricity than the GTX 550Ti. Maximum power consumption for the Radeon HD 7750 is 43w compared to 138w for the GTX 550Ti. See following review: