On 31 July 2019, Valencia moved to England to join Championship club Brentford on a four-year contract for an undisclosed fee, reported to be £1.8 million. Despite making 23 appearances and scoring one goal, he struggled to adapt to the pace of English football during the 2019–20 season. After just one appearance during the opening month of 2020–21, Valencia returned to Poland to join Ekstraklasa club Legia Warsaw on loan until the end of the season. During a season affected by a plethora of injury problems and poor form, Valencia made just 12 appearances and scored no goals. After three months without being named in a matchday squad, he made an early return to Brentford for medical assessment in April 2021. In his absence, Legia won the Ekstraklasa title with three matches to spare. Also in his absence, Brentford were promoted to the Premier League after victory in the 2021 Championship play-off Final.
