Valuing SONIA based fixed for floating swap

Hi. So valuing say a 3m libor fix/float swap is easy as you know what the floating rate will be in advance. Treat as two bonds discount back and differece is the value. But SONIA rate is not termed and is actually last nights rate so how will you now value 3m SONIA fix/float swap if you dont know what the rate will be at pay date?