“Under the entry/exit system, every British traveller to the Schengen area must be finger-printed and provide a facial biometric.” https://www.independent.co.uk/travel/news-and-advice/brexit-border-etias-dover-eurostar-b2130495.html
“The EES was developed while the UK was a member of the EU, ahead of the 2016 referendum.” https://uk.news.yahoo.com/rail-ferry-eurotunnel-passengers-europe-144622282.html?soc_src=social-sh&soc_trk=tw&tsrc=twtr