On 16 March 2021 the Supreme Court heard the Secretary of State's appeal in BF (Eritrea) v Secretary of State for the Home Department.
The case concerns the lawfulness of the Secretary of State's initial age assessment policy under Chapter 184.108.40.206 of the Enforcement Instructions and Guidance ("EIG"). The EIG provided at the material time that claims of asylum seekers to be under 18 should be accepted, unless the case fell into one of four criteria. Criterion C is the subject of BF's challenge. It provided:
"C. Their physical appearance/demeanour very strongly suggests that they are significantly over 18 years of age and no other credible evidence exists to the contrary" 
That criterion was required to be read in conjunction with the Assessing Age Asylum Instruction ("Assessing Age").
The issues for the Supreme Court are:
• What test must the court apply to determine the lawfulness of the Secretary of State's policy on assessing the age of asylum seekers who claim to be children?
• Is the Secretary of State's policy lawful?
The procedural history of the case is relatively complex, but the material background can be stated shortly.
BF is an Eritrean national who arrived in the UK in March 2014. He gave a date of birth of 15 February 1998, thereby claiming to be 16 years of age. BF was initially age-assessed by an Assistant Immigration Officer and a Chief Immigration Officer. They both considered him to fall into Criterion C. Inquiries with EURODAC revealed that BF had previously claimed asylum in Italy and given a date of birth of 15 February 1988, making him 26. Directions were set to remove BF to Italy.
BF continued to maintain that he was a child. Two local authority Merton-compliant age assessments found him to be an adult. A third one, conducted by independent social workers, found him to be child. He was thereafter treated as a child for immigration purposes. BF had been held in immigration detention for a total of 9 months.
In October 2016, BF was granted permission to apply for judicial review on a single ground, that "the published policy… that the [Secretary of State] may treat a person claiming to be a child as an adult if immigration officials think that he or she very strongly looks significantly over 18" was unlawful. BF was unsuccessful in the Upper Tribunal. He appealed to the Court of Appeal.
Court of Appeal
BF's appeal was allowed by a majority (Underhill LJ and Baker LJ, with Simon LJ dissenting).
There was a dispute as to the applicable test. The Secretary of State maintained that the Supreme Court decisions in R (Unison) v. Lord Chancellor  UKSC 51 and In re Human Rights Commission for Judicial Review (Northern Ireland: Abortion)  UKSC 27 only applied to challenges to legislation, rather than challenges to policy or guidance . Underhill LJ did not decide the issue, but concluded:
"…the policy/guidance contained in paragraph 220.127.116.11 of the EIG and the relevant parts of 'Assessing Age' will be unlawful, if but only if, the way that they are framed creates a real risk of a more than minimal number of children being detained…the policy should not be held to be unlawful only because there are liable, as in any system which necessarily depends on the exercise of subjective judgment, to be particular 'aberrant' decisions – that is, individual mistakes or misjudgments made in the pursuit of proper policy. The issue is whether the terms of the policy themselves create a risk which could have been avoided if they were better formulated." 
Simon LJ and Baker LJ largely agreed with this characterisation of the relevant test.  .
The policy was found to be unlawful on a narrow basis.
Two things were noted at the outset. First, that it was impractical for a full Merton-compliant age assessment to be conducted at the initial screening stage . Second, that it would not be desirable for the Secretary of State to be obliged to accept all claims that an asylum seeker was under eighteen . It followed, therefore, that it was not illegitimate for the Secretary of State to have a policy which required immigration officers to make a detention decision on the basis of an initial assessment which would necessarily be confined to an assessment of appearance and demeanour - .
The issue was what margin of error needed to be built into such an assessment, given the inherent unreliability of assessing age by appearance and demeanour. Underhill LJ explained that it was incumbent on the Secretary of State "to ensure so far as possible that such decisions take fully into account the wide margin of error which such decisions will necessarily involve, so that only those young people whose claims to be under 18 are obviously false are detained: in other words, anyone claiming to be a child must be given the benefit of the doubt". 
The policy was unlawful because it did not sufficiently account for that margin of error: the word "significantly" was not enough. Underhill LJ concluded that "only if the officer is made aware of the extent of the unreliability of the exercise, by reference to specific ages, can he or she make an informed decision about whether a claim to be under 18 is indeed obviously false." In the absence of guidance as to the width of the margin of error, there was a real risk of children being unlawfully detained. The policy was therefore unlawful. -.
Underhill LJ did not place any significant weight on the statistical evidence adduced by BF, although Baker LJ drew "considerable support" from it . Baker LJ also concluded that an assessment of age based on physical characteristics and demeanour alone was inherently subjective. As such, without safeguards, such a test would arbitrary, and be an infringement of both the ECHR and EU law. -
The appeal to the Supreme Court
The Supreme Court may resolve any disagreement as to the applicable test, as set out above. The appeal is also likely to consider issues raised by Simon LJ in his dissent. In Simon LJ's judgment, the guidance document 'Assessing Age' was sufficiently detailed about potentially misleading indicators of age so as to mitigate the risks. Thus, Simon LJ agreed with the "real risk" test, but concluded that BF had not shown the policy was unlawful.
 By the time the matter was before the Court of Appeal, the criterion had changed to include a requirement that two Home Office members of staff had separately assessed the person claiming to be a child. In the previous version, this was stated in a later paragraph. Substantively there were no material changes.