Upvote:5
If your visa states that you must be accompanied by your grandparents then it is not valid unless you travel with at least one them. If you want to travel alone you will need to apply for a new visa.
See https://www.gov.uk/standard-visitor-visa/if-youre-under-18 (emphasis added):
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.
If you want a visa that will allow you to travel alone, you need to prove several things:
Travelling alone
You can travel to the UK without an adult (someone over the age of 18).
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
It may be possible to amend your existing visa, but I doubt it. The best way to find out will be to ask at the consulate where you applied for the existing visa.