Depends on the distance from the exchange, be can happily manage 20meg if your close and most llu will do 10meg to a decent distance, of course there's an un-shaped alternative offered by virgin, you just have to take a complete bundle. Virgins traffic management is archaic, but it's just a deliberate incentive to take the higher package.
Besides their shareholders have shifted their pennies into the other, more profitable, branches of the company, there's very little investment in their cable but since there's no real competition at 50meg in their areas, there's no incentive for virgin to stop.