Ok - the exit inventory can only be claimed if it was a term of the tenancy.
As for the £600, you can reject this. They would have to show that cleaning and repairs were needed. Indeed for the repairs, they would have to show that any damage went beyond fair wear and tear.
They can not claim for things which are just worn out
But to answer your question, yes, you can counterclaim, You can sue for loss and damage.
You need to write and set out your losses and request compensation within 14 days or say you will go to Court within 14 days. You should make sure you send this signed delivery and keep a copy.
If they do not compensate you then you can issue proceedings in the County Court. You can either do this online at: www.moneyclaim.gov.uk or by completing form N1 http://s3-eu-west-1.amazonaws.com/hmctsformfinder/n1-eng.pdf and take it to your local County Court.
The Court will then issue a claim which a copy will be sent to the Defendant who will have a limited time to defend it, if not you can enter Judgment and enforce.
If the claim is for £10,000 or less it will be a small claim so you will not need legal representation. Over this value, you would need representation for trial.
Can I clarify anything for you about this today, please?
If not, I would appreciate a 5-star rating for my answer. If you need anything further I am available for a follow up at no extra cost.