Processing Deposit Claim - Deposit registered by Agent on our behalf

Tenancy Deposits

We used an agency for our first letting and they registered the deposit on our behalf. The tenancy has now ended and we feel that the tenant caused substantial damage, costing more than the deposit. To draw the matter to a close quickly we proposed to the tenant that we keep the whole deposit. The tenant has asked if there is an appeal process. The agent is going back to them to confirm there is but also to ask what they see as reasonable.
The agent has said they can manage the dispute process for us at a cost of £120. As I didn't register the deposit personally (it was done by the agent), can I represent myself in the dispute process, or can it only be done by the agent as they registered the deposit? If only the agent can represent me, is this actually a reasonable "professional fee" to include in the quotes of expected costs to put our property back to a similar condition it was at the start of the tenancy? Can I give the tenant the whole deposit back and then take them to small claims court as a alternative to the dispute process to avoid paying the agent? Also, is the agent obliged to provide me with the photos they took at the start of the tenancy? Without them, I'll struggle to prove the extent of the damage tenant done
The tenant don't seem to have complied with the contract requiring an end of tenancy clean, they removed a handle and damaged a fire door beyond repair, replaced the original lock with an inferior (and wrong size) lock, damaged the stair banister, damaged walls, skirting boards and door frames(probably with furniture removal), chipped other doors which have a wood finish, and broke a fire door closing mechanism among other things.