Britons are allowed to stay for up to 90 days at a time and no more than 90 days in a 180-day period.
Long-standing impartiality rules for broadcasters have not been amended recently.
Nigel Farage, Rishi Sunak and Sir Keir Starmer have been featured in videos online.
The SNP and Plaid Cymru are standing for election to Westminster, but only compete in Scotland and Wales.
As polls continue to predict a Labour landslide, what does it mean to control the agenda in the House of Commons?