I assume that it is an assured short hold tenancy because it is a separate dwelling, if it is not a separate dwelling then, although it says is an AST , it cannot be.. If it is, and if he did not protect your deposit and you can prove this, the tenancy is unenforceable.
The correct procedure as you will be aware, is for you to pay up to the end of the term and then, if he doesn't return your deposit, file the dispute. However, I know exactly where you're coming from, the deposit is the last month rent, so he walk away without having paid it, the landlord has not lost anything, and you are not fighting/suing him to get your deposit back. It is very common and tenants do it all the time. Of course, I cannot advise you to do that, but it works usually in practice. Expect the landlord, as you suspect, to come up with all sorts of spurious allegations, but the property was left in a mess, with all sorts of damage.
At least then, he is suing you rather than the reverse and of course, he's unlikely to know that you have the copious photographic evidence that you do. Take particular note of bathroom and kitchen fittings and floorcoverings, because they are a common source of alleged damage.
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