AST

Guarantor

I have a tenant with a guarantor named on the AST contract and this second year,
we will renew the contract and she wanted her father as guarantor out of the contract, and she wants her name only on the contract. Credit referencing is the
issue as the letting agent introduced me to this tenant and I believe she has no enough credit to apply on her own. She paid full rent the first year and I extended it to another 6 months because I gave her notice to increase the rent, and again paid in full. Now come this renewal, and she says no guarantor. I told her, ok no guarantor but she has to pay me in full rent the whole year. We agreed that she will pay me biannually, meaning she will pay the 6 months rent in full on signing the contract and another 6 months rent after six months.
I put a clause in the contract that if she does not pay 6 months in full after six months, the contract will be null and void. Is this enforceable if the tenant goes in default.