Does anyone know if I would be able to use a BSOM to read a 2.5Khz pulse output from a flow meter or how I could go about doing it.
Yes, since you only need to count pulses, 2.5 kHz is able to be read using attachInterrupt on a Gen 3 device. The difficulty is that the nRF52 MCU has variable interrupt latency, so measuring pulses is error-prone, but counting works fine.