Builder's tea, AKA builder's brew, is a British term for strong tea, although usage varies. Builder's tea is tea drunk by a workman on the job, and therefore is somewhat less fancy than a proper cup of tea, always made by a tea bag, which is usually placed directly in the cup; it is usually flavored with milk, maybe with sugar, no lemon.
Because internet, there are a number of "recipes" for builder's tea posted on a number of blogs. There seems to be a general consensus that builder's tea is strong, but this appears to mean black tea, one teabag, steeped to taste, rather than the American interpretation of strong tea, which tends to involve either multiple teabags or leaving one bag in for much too long. There is also a frequent consensus that builder's tea is usually "milk and two" (milk and two spoons of sugar), but many people will hastily then go on to say that they don't use two spoons -- almost everyone seems to prefer one or none.
One aspect of builder's tea that, as an American, I blithely read over in the first three recipes: you can drink it from a mug, rather than a proper teacup. This slightly barbaric habit appears to be part of the charm of the concoction.
The term often sounds dismissive and condescending, and sometimes it is; however, this is generally seen as a perfectly acceptable way of taking tea, if you are in a hurry or if you just need something hot and caffeinated during the work day.