Just arrived in the UK on Eurostar and all passengers had to go through immigration and ticket checks upon arrival. It was unclear what was actually being checked. There were 2 narrow lanes and one wide lane set up in the corridor at he bottom of the travelators. It was unclear what the lanes were for as the overhead display screen's font size was too small to read. By the time one passed the screen, it was too late to go into another lane. Each lane led to two officers, thus the wide lane moved twice as slowly as the narrow lanes. The officers on the right were Eurostar agents (much like those doing the UK "exit check") while the other officers were UKBF. The Eurostar agents glanced at people's passports and then waved them through very quickly.The UKBF officers were asking for tickets and comparing passport photos to faces carefully. I overheard an officer being questioned and he said that the officers in Paris/Brussels did immigration while he was doing customs. This was a blatant lie / fob off as customs involves checking baggage and customs agents at the airport don't ask for passports (unless you are caught making a false declaration). Also HMRC should be performing customs checks rather than UKBF, and UKBF really has no business checking tickets (they don't do it at the airport). Perhaps this is a trial run to see how long it takes,w and they didn't actually check luggage since we haven't left the EU yet. I was off the train quite early so I wasn't massively inconvenienced, but those at the back must have been delayed some time. I will be avoiding Eurostar from now on unless there are more positive reports in the future. I'm prepared for this when flying, so don't mind so much but I expect to be off a Eurostar and onto thameslink in 5 mins.