I think maybe they just lost their way...

Pre-war and just afterwards they company tried to produce the highest quality air guns made to firearms standards.
They cost a fortune to make and had a high selling price.
Rumour has it they were losing money on every Service rifle and Senior pistol they sold...
So then they tried cheapening their existing designs to make money and the trouble there is that once you've offered a high quality product and then try to take it away - the public can see what you've done and expect a lower selling price.

Then add to this the timing...
The war had not long finished, companies had bugger all money to invest, machinery was knackered from years of wartime production and the labour pool was in a state of flux.
Contrast this with Germany - who had all their designs nicked by the British as war reparations and had their factories flattened.
So they ended with new factories and came up with new designs being made on new machinery.

Meanwhile in Blighty the 'make do and mend' mentality was firmly entrenched while the customers were patriotic to start with this quickly changed as the next generation came along who were less tethered to the past and whilst their employment might be governed by the old guard, their spending most certainly was not and the new German rifles were very well made and better than the by now, old British offerings.