The OP says the guy just tapped it, not smacked it hard with a whopping great sledgehammer.
Given that, there’s no way that tapping the axle lightly would drive the BB cup out of the frame stripping the threads in the process.
Conclusion: the threads were already stripped by whichever numpty previously worked on the bike (you say it’s an ex demo) and had been lightly reassembled.
Your mechanic definitely does not owe you a new frame as some have suggested (unless you bought the bike from him/that shop).
Solution?
First, get the BB threads rechased, either by the shop or by your local machine shop/engineering shop/agricultural engineers.
If that doesn’t work, contact Specialized UK and see what they suggest. Unlikley they will warranty replace it given that it’s the result of damage, but I have found them very helpful in the past.