From 2 April, Swedes and other EU citizens must apply for an ETA (Electronic Travel Authorisation) when travelling to the UK. This means that Swedes who want to visit England, Scotland, Wales or Northern Ireland must apply for an ETA before they start their journey. A UK ETA costs £10, equivalent to around SEK 130, and can take up to three days to issue.

If you do not have an ETA, you will probably not be allowed to board the plane to the UK. EU citizens can apply for an ETA from now on, which can be done on the website below. However, if you are travelling to the UK before 2 April, you will not need an ETA.gov.uk

Source.Thick