British Traditional Wicca sounds like an ancient practice, but it's no older than any other kind of Wicca. It's a more general name for a few different Wiccan traditions that come from Britain and are the original branches of the religion.
Typically, the term "British Traditional Wicca" would refer to either the Gardnerian or Alexandrian traditions, as they are the original ones. Many smaller offshoot traditions that come from these two can also be classed as British Traditional Wicca, such as Central Valley Wicca and Blue Star.
The key is that any coven claiming to follow a British Traditional Wiccan path must be able to tie itself through initiation to Gardner's original coven.
Since his form of Wicca involves oathbound and secret material, only people who trained with another BTW coven would know the innermost details of their faith and rites.
That also means that I don't have all the details about British Wicca either, since I am not a member of an initiated coven. But I can pass along the basics, which are the same for all the various forms of Wicca that have developed over the past 60 years since Gardner's day.
Of course, that's just a rough outline of what's involved. You have to be part of a BTW coven to get all the inside details. This form of Wicca is fairly organized and not the "follow your heart" kind of Wicca that many other traditions prefer.
The coven will be divided into levels or degrees (usually 3), and you have to do a substantial amount of training and learning before moving up. So even within a coven, there can be secrets and hidden mysteries from one degree to the next.
So that's the general idea of British Traditional Wicca. If it doesn't sound like your cup of tea, there are other ways and styles of practicing Wicca these days that might suit you more. Several of them are linked from the Wiccan Traditions page, so check them out.