Although much of this will be down to the particular contract and agent, common sense should prevail.
The government has brought in temporary measures to prevent evictions, however your situation is slightly different as you are quite happy to move out. Given this, I would have thought that any reputable agent would want to work with you to minimise the cost for their client - the landlord. Contracts are contracts, however most agents (I was one myself for many years) will try to help you. The 11% is simply their fee that gets paid to them by the landlord - interesting to see that they seem to be saying that even if they find a replacement tenant straightaway, you still have to pay their fee until the end of your tenancy, even though you have moved out and the landlord has a new tenant. This means that effectively they are being paid twice!!
The tenant fee ban last year did away with tenants costs, so I would argue that this should not be counted, and yes, of course you would be liable for the inventory costs anyway.
Overall I would suggest that you have a sensible conversation with both the agent and your landlord, explain the situation and try to come to an amicable solution. As a landlord myself, I just want happy tenants as it makes life so much easier for everyone. If you propose to them that you will allow them to re-market the property straightaway, give them access to viewings whenever they like, and make sure that the flat always looks great then they should be able to let it out fairly quickly. You're much better to just be upfront and try and work with everyone in my opinion.
Feel free to PM me if there is any more in depth information you need.