UK ,Can Landlord charge extra for ?

You would need to PROVE a mold problem existed before you rented. Why would you rent a place that already showed an existing mold problem? This is what a judge would ask you. Yes, a landlord can charge you more & take you to court to get it if you don't pay it, plus keep your deposit.

They can charge you for resolving issues that cost more than your deposit.

If an issue was pre-existing, you should have documented it (in the inventory / check-in process, or by asking the landlord to fix it)

Damages and lack of cleaning by the tenant they can charge you fo and if you don't pay they can take you to court and sue for the money over and above the deposit......... however if you do not agree you can make a claim via the deposit insurance scheme https://www.gov.uk/tenancy-deposit-protection

Damages are not limited to the amount of the deposit.

So I suppose that you can prove by copies of letters to the agent/landlord that they knew about a mould problem and they took no action? If it were (say) a dripping pipe joint, and you knew about it and ignored it, then the LL can naturally sue for the damage from the result of your omission. If he knew and did nothing, that's on him.

They can go after you for all damages above and beyond the deposit, yes. If they couldn't, deposits would be ridiculously high.