ETAs will be mandatory for travel to the UK and are being introduced in several stages for different nationalities, over the coming months.

The following link is the best overview, where visitors can find official information and apply: https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta

Another helpful link, which details key deadlines for groups of visitors (by country), is as follows:

https://www.gov.uk/guidance/check-when-you-can-get-an-electronic-travel-authorisation-eta

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.