USA - NY Times May 2018 says this www.nytimes.com/2018/03/30/world/americas/travelers-visa-social-media.html
"Nearly all applicants for a visa to enter the United States — an estimated 14.7 million people a year — will be asked to submit their social media user names for the past five years, under proposed rules that the State Department issued on Friday."
I don't know if that includes the UK - but you certainly need to fill in that annual visa waiver document in advance in order to travel from UK to US so it possibly will apply.
"The proposal covers 20 social media platforms. Most of them are based in the United States: Facebook, Flickr, Google+, Instagram, LinkedIn, Myspace, Pinterest, Reddit, Tumblr, Twitter, Vine and YouTube. But several are based overseas: the Chinese sites Douban, QQ, Sina Weibo, Tencent Weibo and Youku; the Russian social network VK; Twoo, which was created in Belgium; and Ask.fm, a question-and-answer platform based in Latvia."
Okay sorry I read on and does not so far include UK (although if they are questioning you at immigration they still might well ask for it)
"Citizens of roughly 40 countries to which the United States ordinarily grants visa-free travel will not be affected by the requirement. Those countries include major allies like Australia, Britain, Canada, France, Germany, Japan and South Korea."