It might be the Beaufighter anyway.Had the 12Y been in production in Britain instead of Rolls trying to reinvent the wheel with the Peregrine then the Gloster Reaper would have been more likely to enter service instead of the Beaufighter,
Admittedly, the G.39 did fly a few months earlier than the Beaufighter IOTL.
However, Gloster's factory re-tooled to build Hurricanes in 1939. It would build 2,750 of them between November 1939 and March 1942. 1,211 of them were built in 1940. Gloster also delivered the first of 3,300 Typhoons in December 1941. It would have re-tooled a year earlier, but the Ministry of Aircraft Production decided that it was better to keep the Hurricane in production for longer.
Meanwhile, Bristol delivered its last Blenheim in January 1940. The first batch of 78 Beauforts was delivered between April 1939 and March 1940. The first 150 Beaufighters were delivered between April 1940 and March 1941. AIUI the Beaufort and Beaufighter could be built on the same production line, which AIUI allowed Bristol to complete the first contract for 150 aircraft sooner than an all-new aircraft.
Therefore, I think that they'll still build the Beaufighter because it doesn't disrupt Hurricane production at a crucial time. Putting the Reaper into production in 1940 would disrupt Hurricane production at a crucial time.