Under US rules, SEC Rule 15c3-3, broker is allowed to rehypothecate up to 140% of your debt’s value. For the securities it’d rehypothecate (loan out) you’d lose SIPC protection as it doesn’t cover counterparty risks
While googling about it now, I’ve noticed that it looks like in UK (and you’re dealing with IB UK) there’s no similar regulation and so maybe as a IB UK customer you have higher risk with regards to it… i don’t know much about it, i’ll need to read up