Gayot Fow, our much-missed member-on-sabbatical, has answered this on his blog. I post the text of the answer here for archival purposes; everything below the line is his, reposted with permission. (For the benefit of the OP, whose first language may not be English, I should note that one of the few acronyms Gayot has not defined, "CYA", stands for "Cover Your Behind", and refers to behaviour designed to deflect potential criticism from a superior).
A minor (someone under the age of 18) has been issued a Standard Visitor Visa (SVV) and, according to law, the adult responsible for accompanying the child is identified explicitly by name, nationality, and passport number in the form of an amendment. This amendment is done by an Entry Clearance Officer (ECO) or an Entry Clearance Assistant (ECA) and then inspected by an Entry Clearance Manager (ECM).
When this type of visa is issued, the entry clearance itself is pasted to the recto and the accompanying adult(s) are listed, in handwritten ink, on the middle of the verso. Then two vertical lines are written on the verso from top to bottom broken in the middle where the name(s) are written (i.e., the entire verso is used up). This practice prevents additional name(s) from being added without exposing fraud or forgery. Corresponding entries are made in Proviso (the entry clearance system) and their internal visa security system.
When the child arrives in the UK, the Immigration Officer (IO) checks everything to be sure the accompanying adult has been identified on the child’s visa. On the first visit, it is likely that the IO may take the relevant passports to a secured workstation to double check that everything is in order so it’s reasonable to expect a momentary delay wherein the IO seemingly ‘disappears’.
All of this diligence (and more) is a result of Section 55 of the Borders, Citizenship and Immigration Act 2009 (variously referred to as ‘The Children’s Act’, or ‘Best Interests of the Child’, etc). This is a responsibility that the Home Office has taken with ponderous diligence, their guidance is published at Every child matters: statutory guidance and various brochures elsewhere.
The responsibility to protect children extends not just to visa holders but to British nationals as well and you can find the occasional anecdote where a British family travelling with children has been selected for secondary inspection. These anecdotes expressing outrage and indigence can be met with a single answer: Parliament has told the Home Office to carry out these responsibilities and if you don’t like it, that’s tough titties, write to your MP and see what happens.
Enough digression, returning to the main point… What happens if a child’s visa needs a variation? And specifically, what happens if the variation entails adding (or removing) from the list of authorised adults? In the case at hand, a teacher wants to organise a school trip where some children already have long-term SVV’s that identify an adult who will not be accompanying the child. If the airline is diligent the child will not be permitted to board the aircraft and if the airline fails to be diligent, they will be fined and landing IO will have the child removed (i.e., bounced) and the teacher will be in serious trouble and likely removed also (thereby invalidating the entire class’s visas).
And what’s the answer to this conundrum?
Cancel the child’s existing 10 year SVV and issue one just for the class trip? And presumably apply for another 10 year visa when the child returns? 10 year visas are breathtakingly expensive and taking this approach places an unfair burden on the child’s parents.
Can UKVI not just issue another visa that runs concurrently with their existing visas? Flash back to the 2006 – 2008 era when Liam Byrne was Minister of State for Immigration, he introduced a sensible policy aimed at curbing abuse that a person could have only one form of permission at a time. While there is no law governing this policy, it has worked for a long time and is unlikely to be disrupted for the case at hand.
In fact, the answer is indeterminate! In fact it is down to the ECM at each post to decide how to handle it. Among the alternatives available to the ECM are: a) issue a tour operator visa listing all of the students with a footnote containing the names, passport numbers, etc of the students travelling on an existing visa; b) issuing a letter to each student to present to the carrier and the landing IO; c) any other approach that the ECM deems appropriate, including the approach that the previous ECM used (this is the most likely scenario). He can use smoke signals if he wants (as long as it works and he can show it’s reasonable).
If the ECM is especially cautious and wants to follow CYA protocols, they can make a referral to the Sheffield Office and get advice from there. In that case it will be down to the unit in Sheffield to consider how many people are affected, the nationalities involved, the credibility and standing of the teacher, and whatever else they want to assure that their responsibilities to Parliament are painstakingly discharged. If they deem it necessary (which I doubt), Sheffield can also issue guidance to all ECO’s globally in which case it will be published and available somewhere on the net.
Why don’t they cover this in a law somewhere? This is very unlikely to happen. Parliament, the British public, and the UK legal community are all content with broadly worded laws that are vague in some areas. Both sides of the coin benefit and it’s not Parliament’s job to clarify what they have enacted in the first instance. Also, it is not the Home Office’s job to explain what Parliament intended, but this is a matter of raging and spirited debate often involving the courts.