Out of curiosity, why do you think replacing a static mortar pit with a mobile mortar be any less of a blob deterrent?
Mortars in general are never a proper counter against blobs. Normal mortars are not flexible enough to respond to incoming mobile blobs due to low range (which requires constant repositioning) and a very long time between 'enemies spotted' and an 'effective load delivered (setup time and rate of fire)'. The 120mm has a good AoE and range, but suffers from long reload and setup time (regarding countering blobs).
However, the British mortar is able to respond early enough to incoming threats (no setup time, long range) and deliver a massive load of firepower (high RoF, 2 mortars).
The British mortar is a defensive tool whose max range should not extend further than the frontline. That also ensures the response time to threats because it won't be busy doing offensive tasks.