It’s most unlikely that you would be bound by this guarantee unless you had actually signed a physical document.
It may be that you made an electronic agreement by clicking a link but before any landlord or claimant could enforce any payment against you, they would have to do prove to the satisfaction of the court that you had agreed to this.
If this is an Assured Short hold Tenancy, then the guarantee agreement is not enforceable against you unless you have been provided with all the documentation, including the tenancy agreement and a copy of the guarantee agreement, which the tenant receives.
As it seems you had none of that, then it’s most unlikely that this would be enforceable against you.
Can I clarify anything for you?
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In which case, I cannot see how you could possibly be liable under the terms of the guarantee. Don’t be surprised if the agent tries to enforce it, but this would not stand up in court based upon what you have told me.