Brexit didn't actually go through until after the 2019 election. During which Boris Johnson campaigned and made many promises about Brexit, eg it was "oven ready" and absolutely would not be a hard Brexit. They claimed to have had better trade deals ready.
There were plenty of promises made. Boris was even put on trial in 2019 for his 2016 Brexit campaign lies, where he only got off on a technicality because the Brexit campaign wasn't technically part of his ministerial duties. Everything since the 2019 election was, though, yet he was never charged again - even when he was caught lying to the queen.