A new report from Kantar Media estimates that the value of the health and fitness wearable technology market in Great Britain is between £225 million and £375 million, or $360 million to $601 million. They estimate 13.1 million British people will be using health and fitness wearables by 2015, more than double the 6.7 million people (or about 10 percent of the country) tracking metrics like steps, fitness levels, and heart rate currently.

Kantar's market analysis includes free apps, paid apps and devices. They surveyed 2,000 participants aged 16 and older throughout Great Britain. According to the data, half of the 6.7 million users are only using free apps. Devices make up 79 percent of the remaining, paid half -- about 2.6 million users. Accounting for 7 percent of the market that uses both devices and paid apps, that leaves 28 percent, or about a million users, using only a paid app for tracking.

The survey respondents who used mostly free apps did express an intention to purchase mobile health devices in the future. Eighty-four percent said they were likely to purchase either devices or paid apps in the future. And 88 percent of participants who said they planned to use wearable technology in 2015 said they would likely purchase a device. 

Kantar also found evidence of a growing interest in glasses products like Google Glass. Fifteen percent of people already using wearable technology for health and fitness expressed an interest in smart glasses, as did 30 percent of the group that planned to use wearable technology next year. They expressed interest in other kinds of smart wearables as well: 12 percent of current users and 21 percent of 2015 users saw themselves buying smart jewelry, 22 percent of current users and 32 percent of 2015 users expressed an interest in smart clothing, and 27 percent of current users plus 30 percent of 2015 users had their eye on smart shoes.

Kantar's numbers are actually pretty close to the number of wearable adopters in the United States, according to a recent PricewaterhouseCoopers report. PwC said that 21 percent of US adults used devices (their report did not include apps), which amounts to about 6.6 million users. Kantar itself previously found that 55.7 million American adults used diet and fitness apps in 2013, up from 43.9 million in 2012.