The big thick head is achieved by serving the beer through a sparkler - the bit at the end of the spout. This forces the beer through a number of holes which can be made larger or smaller by tightening the sparkler. The smaller the holes, the more the beer is agitated and the foamier it becomes. The effect can be accentuated by using a swan-neck, a long, curved spout enabling the beer to be served straight into the base of the glass.
Some beers, like Boddingtons and Tetleys, are brewed in such a way that they are not harmed by this treatment. Basically, they are over-bittered to enable them to withstand the bruising from a tight sparkler. Tetleys served flat tastes more like its name-sake, tea.
Fortunately several local brewers like Greene King and Adnams have refused to become fashion-victims and do not use these devices in their own pubs. In the free trade and elsewhere they have no control. Charles Wells do use swan-necks, albeit with the intention that the sparklers are set loose. Most local Charles Wells pubs do not use sparklers at all. The worst offenders are the big brewers' pubs, especially managed houses where sparklers and swan-necks are used indiscriminately regardless of the style or character of the beer concerned.
So what can you do if you do not want a thick head? You can always ask the bar staff to loosen or remove the sparkler and most will oblige. If you feel you shouldn't have to ask, take your custom elsewhere. There are still plenty of pubs around which treat beer as more than just a fashion accessory.