I remember that two years ago, passport holders of non-EU nationalities were required to present a chest X-ray along with accompanying report on arrival at Heathrow airport. (And if you didn't have one, then you had to wait in queue for hours while they got a chest X-ray done for you.) That was back when there was a bird flu scare going around. Does the directive still stand or has it been lifted? I tried looking up but couldn't find a definitive answer on the situation at present.
As a UK Gov directive it is still applicable.
A person who intends to remain in the United Kingdom for more than 6 months should normally be referred to the Medical Inspector for examination. If he produces a medical certificate he should be advised to hand it to the Medical Inspector. Any person seeking entry who mentions health or medical treatment as a reason for his visit, or who appears not to be in good mental or physical health, should also be referred to the Medical Inspector; and the Immigration Officer has discretion, which should be exercised sparingly, to refer for examination in any other case
This internal guide for use by entry clearance staff (Last updated 10 March 2010) states
Advise the applicant to carry the original TB certificate (and their chest X-ray) in their hand luggage for presentation to an immigration officer on arrival in the UK to prevent delays or referral to the Port Medical Inspector (PMI).
Whether it is followed in practice at LHR, I'm not sure.
I did travel to UK on a Work Permit between 2002 to 2006 and was not once asked to present the X-Ray even after clarifying my long term stay.
According to various local forums in Russia, this practice was in the past just to check some people for MTB. Even in the past, not all the people must do such test, and many of problems arose when the X-ray was broken.
In general, only people who going to enter the UK for more than half-year must go to the test. According to current situation, in all the terminals of Heathrow there is no X-Rays tests.
Unfortunately, can't find any official information for this.