Watneys was a British brewery that peaked in the 1930s and merged with Grand Metropolitan in 1972. The brand was closed in 1979. Grand Met later merged with Guinness in 1997 to form Diageo. Watneys beers - Special Bitter, Red Barrel, Mild, Light and Brown Ales are no longer brewed under that name.
You can still buy brown ale, and there are some pubs, normally those with micro-breweries, that still sell mild. I hope you can find one for your dad.
One of the best bits about a brown and mild (or a light and bitter) was that the barman would get a pint glass and put the mild in first. It was always more than half a pint and you were then passed the brown ale to pour into the draught beer. On a good day, you'd get a pint and a quarter.
Then bloody Weights and Measures got involved and started prosecuting pubs for serving MORE than you were paying for!!!