GOV.UK guidance for someone under 18 applying for a Standard Visitor visa is:
If you're under 18
You can apply for a standard visitor visa if you’re under 18 and:
- you’ve made suitable arrangements for your travel and stay in the UK
- you have consent from your parent or guardian to travel to the UK
- you’re able to pay for your return or onward journey
- you have enough money to support yourself without working or getting help from
public funds, or you have family and friends that can support you
You can travel to the UK without an adult (someone over the age of
Your parent or guardian will need to provide their:
- written consent for you to travel to the UK
- full contact details
They’ll also need to provide proof that you have somewhere suitable to
live during your stay in the UK, including:
- the name and date of birth of the person that you will be staying with
- an address where you will be living
- details of your relationship to the person who’ll be looking after you
- consent in writing so they can look after you during your stay in the UK
Travelling with an adult
When travelling to the UK with an adult (someone over the age of 18),
you’ll need to identify them in your visa application.
If the person you’re travelling with isn’t your parent, you’ll need to
provide specific information about them in your application.
Their name will appear on your visa, and you’ll be refused entry to
the UK if you arrive in the UK without them.
You can identify up to 2 adults in your visa application, and your
visa will only be valid if you travel with at least one of them.
The adult can apply for a visa at the same time, but you must each
complete separate applications.