If you are moving out on 18th then if you hand back the keys/leave them in the property, as your contract ends on 19th, there is no additional day. You have no need for the additional day, they do because they can't provide you with a check out appointment on the day your contract ends.
In short, their problem.
Your aim is to return the property to the level in which you received it, deducting fair wear and tear over the period of time you lived there.
If you lived there 6m, it ought to be identical to how you received it. If you lived there 6 years say, it's another story and actually, given that rental properties ought to be decorated every 7 years give or take, and the value of carpets etc would be pretty much depreciated in around 10 years, to be flippant, so as long as the house is still standing and there are no holes in the wall, you are probably safe to assume that you are going to get the vast majority of your deposit back.
Your deposit is YOUR money. If the LL wants any of it, they have to prove they have a right to it.
Your inventory will have stated condition at the time of your tenancy start. This is the level to which you are working to. The inventory provides the benchmark to which you work. It is a documentary snapshot and needs to provide detailed info on what condition the property was in when you got it. If it does not, then the adjudicator can not support any claim the LL makes.
You are not expected to BETTER the property, and any claim the LL has needs to be shown as something they would lose financially as they would have to put right to re-let. I had a LL trying to claim money for damage when they had sold the property. the decision was made to reject their claim as the LL had not lost anything in real terms as they had sold the property as it was.
Was the house professionally cleaned for you to move into? If so, you will need to have it cleaned to that same level.
The LL intention to fully refurbish is potentially irrelevant, given that they can change their minds, so to protect yourself and minimise additional charges, i would strongly recommend the following:
IF THE INVENTORY STATED IT HAD BEEN PROFESSIONALLY CLEANED SPECIFICALLY FOR YOUR TENANCY have the house cleaned professionally, including carpets. If the property was not cleaned professionally, and the inventory states this, or there are lots of comments like Surface dust or cleaned to a domestic standard, then this is the standard to which you can be held. No higher.
If you do need to have the property cleaned, one idea is to use the cleaning company the agent works with regularly. That way if there are any cleaning issues, it will be the responsibility of the cleaning company to put right as they know the standard to which they should be working. the Agent will also not be able to dispute the professional status and will be happy with the receipts provided. This will satisfy the adjudicators that the property is returned in the same state from a cleanliness point of view.
When you have left the property make sure you take photos of everything, check the tops of door frames, skirting boards, doors etc for dust. Make sure the oven is cleaned, the hob and extractor.
Make sure that if there are any stains or marks that you photograph these with something in situ so you can provide perspective and a scale.
Then leave, notify them in writing that you have left the property, the keys are there (or return them to their offices) and that you are no longer able to access it and won't be liable for the additional day.
As a former clerk, I lost count of the times where the agent was clearly showing the property to potential tenants before the previous tenant had had their check out and therefore i was unable to ascertain who had left dried grass on the carpets etc. If the tenant has images taken when they leave, they can protect themselves from anything happening after they leave.
If there is no inventory prepared specifically for you, or no inventory at all, then no deductions can be made as original condition of the property can't be established.