Fireflys handle AT well enough.
On a Crossing in the woods- or Road to Kharkov'ish map, yes, but on giant maps or maps with many sightblockers, the firefly really starts to fall behind and show its issues. It's too slow to escape a tank flank attack. And have you tried flanking an Elefant with fireflies? Good luck.
Massive panther, ele, jp4 gameplay is a big threat in 2v2 and upwards, the last thing the brits need is more reliable anti infantry tanks. :/
They already got cromwells, centaurs, and all the Churchill variants vs inf