I used small doses everyday for years, it's the only thing that made my social disorders bearable. I wasn't breaking any laws.
Why did the (UK) government recently ban phenmetrazine when they formerly tolerated its use?
As correctly asserted, it is banned class B stimulant un the UK. In fact it was banned in 1971 so I am unsure why you think the ban was recent.
If you have social disorders and live in the UK you can ask for a referral from your doctor. Most common route is medication coupled with therapy (group or solo).
To why it was banned is a deeper question. When the misuse act was drawn up it pulled together substances that were known to be used to a 'high'. Primarily in Sweden it was abused for the high it gives leading to classification as a narcotic in 1959