As a French who lives in London, my limited experience (London - Brighton mostly) is that they're fine, reliable, and cheaper than the French TGV (which is generally awesome and uncomparably faster, but ridiculously expensive)

I have a feeling the question might be linked to some common myths about the UK being spread by the French left for ages (evil privatized Thatcherite trains crashing into each other, third-world healthcare with people dying in droves in hospital corridors, etc). Right ? :)