A few weeks ago, I did exactly what you're telling us all to do and moved about £20k of my PBs to some funds on the Vanguard platform in a S&S ISA.
I've just had a look, and in the last few weeks, I've lost nearly 15% of the value, or around £3k. Now, I'm not worried because that money is going to sit there for at least 10 years as part of my pension provision.
But if I needed that money today to pay for a new car, replace the roof or pay the bills if I can't work, that would cause quite a difficulty.
I've done quite well with PBs over the last few years, with returns of 2.1%, 6%, 0.6% (OK, not great but that was last year, and it matched the best available instant access savings account) and this year I've already won £200 on about £30k, so again, no worst than instant access savings.
Now I'm probably going to move more towards investments, along with mortgage overpayment as this is finally looking like it might be worth doing on our long term base rate tracker, but it won't stop me keeping a good chunk in PBs or another cash vehicle to avoid needing to access investments at the wrong time, because now is definitely the wrong time for me.