Of course a provided machine should be clean and fit for purpose.
Op seems to suggest she has spent hours cleaning this seal and it is still caked in dirt. The dirt will either come off or it won’t. If she has spent hours on it already, then it’s either just stained, but no longer dirty, or it’s an unremovable dirt. We cannot tell. People do have different tolerances for things like dirt or staining and what one person will happily accept as a used item that isn’t pristine but usable, another will describe as disgusting. That’s another reason why lots of LLs don’t provide white goods…lots of people really do want new and don’t like pre-used goods and want the LL to magically make them look new. Of course there are shit LLs too who provide inadequate white goods and aren’t interested in dealing with them not working etc.
I maintain that the approach to take wit the LL if you want action, is to not talk about the seal or the dirt…quite simply to focus on it not cleaning clothes as is the purpose of washing machines. If the LL provided a machine as part of the contract, it must clean clothes. If it doesn’t at the start if tenancy they will need to sort it out, whatever that means. If the item is stained in tenants view but perfectly workable, there is more scope for dispute about whether there is a genuine issue which needs addressing, or it’s a matter of different tolerances over used goods.
If you want the LL to take action, just tell him clothes don’t come clean with it. If you aren’t prepared to say that, and you aren’t happy with the seal, then it might fall to you to do something about it and replace it (I’d think this is actually what LL will do if you focus on it NOT WASHING rather than being dirty) or to choose to get your own machine.
So I’d say, with being a tenant, you both have to be firm and clear about items that don’t work which are the LLs responsibility to provide and to fix, and also a bit proactive in sorting out some basic things yourself. For example, I knew a LL who said the tenant rang late at night when bulbs had blown, when the key was stiff in the door and needed a squirt of WD40 and about a solitary broken curtain hook.
I totally agree that the LL should supply what is in the contract and it should be in good working order. Challenge it, if it isn’t. Also be prepared to address yourself little things, that are either a question of taste and tolerance, or are minor minor home maintenance like bulbs, rather than behaving like a child with a LL parent.
It’s hard to see from a thread like this, if this is a question if something not working, or tolerance of used condition. Op says the item is disgusting and dirty. She also says she has spent hours already cleaning it and it’s still dirty - so on one level she felt she shouldn’t have to do this (understandable) but on the other hand, has out the work into it, although it doesn’t seem to have worked enough. It’s unclear if this really is an item that can come clean whether it’s by her or someone else’s effort, or if this will be the solution anyway. But she doesn’t seem to want to take the route of ‘it doesn’t wash properly so doesn’t work’ - this would be the simplest way to get the LL to come and look again and resolve it.