In order to travel to the United Kingdom, there are certain entry requirements you must meet depending on the country you live in. Some visitors may require a visa, and some may require an Electronic Travel Authorization (ETA)
You can check if you need a visa and if you meet basic eligibility requirements HERE.
Electronic Travel Authorisation
ETAs are now mandatory for travel to the UK for those who do not require visas.
The following link is the best overview, where visitors can find official information and apply:
https://www.gov.uk/guidance/
Another helpful link, which details key deadlines for groups of visitors (by country), is as follows:
https://www.gov.uk/guidance/
Each individual visitor is responsible for ensuring they have their own correct authorisation to enter the UK. As such, we are not able to provide any individual advice, but would encourage you to direct queries to the official links above, which also include the option for visitors to web chat with the appropriate authority.