MMB report failed because it naively thought that you could introduce controls over what brewers sold in their pubs. Some old duffers in parliament thought that you would get far more old codswallop in tied houses. The big 6 saw the light, worked out that you could make more money in other areas of business, even soft drinks, bought each other up, sold out, allowed pub chains to fill the vacuum and here we are.
Can't turn back the clock now, why not just let interbrew (or its even newer and bigger next stage, just own everything). Well at least they got told off for pretending that they were a family business (recent Stella ad).
The funny thing is that Watneys, and yes it was bad, now brew Budlighter under license