You have two goals, which can be pursued at the same time: getting the landlord to fulfill a legal obligation, and getting the landlord to suit your personal preferences. On both items I think you will be well-served by presenting these as reasonable requests, which it would only be reasonable for your landlord to satisfy.
The key to the former is going to be to express your desires as such (not as obligations on your landlord), recognizing that you are requesting something they don't have to provide. You can offer everyday sorts of reasons for this:
I often make arrangements with friends to come over, but I don't want to interfere with you showing the place. If a showing is cancelled, could you let me know so that I can go forward with my plans?
Under no circumstances should you treat your preference for being present during a showing as something that your landlord needs to accommodate and account for-- it isn't, and if they don't care, arguing based on an obligation that your landlord knows doesn't exist puts you immediately in a terrible spot, making you seem somewhat entitled and willing to be less-than-honest to get what you feel you are entitled to. This is especially the case if you're intending to hold the landlord to the letter of the law with notice periods.
Ultimately, you are not entitled to a considerate landlord. If they just don't care about being considerate towards you, you aren't going to be able to force them to change. The most you can do is ask.
The key to the latter is going to be recognizing what legal options you have for a remedy to insufficient notice, understanding which of those you might actually use, and being prepared to follow through. Also, it's worth recognizing that the landlord may not have much more notice than you (though they are still required to schedule showings such that you can be given proper notice).
I'm not familiar with UK law, but depending on what type of notice the landlord is required to give you may have a hard time proving that you didn't get it as early as you should have. Even if you can, the legal remedies may not be what you imagine. If your landlord has to pay a 50 pound fine for each violation, that may encourage them to give proper notice before a showing. If the remedy is that you are allowed to leave the apartment without penalty before the lease is up, then unless you're ready to move out the law may not do much for you.
Since legal action may end up being inadequate or too extreme, and the threat of legal action is fundamentally adversarial, I suggest asking the landlord to provide sufficient notice while mentioning the legal requirement. That's a reference to and a reminder of the law, not a threat:
It can be hard to shift my schedule around to make sure the place looks great for a showing, so could you let me know at least 24 hours in advance, like the lease says?
You are entitled to the protections the law affords you, but that being true doesn't magically provide what you want (obviously, as your landlord isn't following the law in this case). But you generally have to engage the legal system in some way to enforce your legal rights.
Whatever you choose to do, the first step should be to adjust your perspective a bit, because coming at your landlord from the wrong angle all but guarantees a difficult relationship. Your landlord may or may not (and it seems like "not" here) care about your personal contentment compared against their own business concern. I'll take quotes from the question, to illustrate what I mean.
- Taking over "your" house
surely if you want to take over someone's house for a day, you give them a little more than 24 hours notice...
It's not your house. It's your landlord's house, which you rent from them. To you, it's a temporary home. To your landlord, it's an ongoing business. To maintain their business, the landlord needs to show it to potential renters, and to avoid losing income between leases that's going to overlap with existing tenancies.
Further, you are not required to do anything for these showings. You may choose to do various things, but you can also continue on with your ordinary living activities whether there is a showing or not. That the landlord is legally required to provide at least 24 hours' notice is an important issue, but it's a separate issue from this.
- "Forcing" you to stay in for periods of time
they failed to notify us when a viewing was cancelled and made me stay in unnecessarily, and they made me stay in the whole day today as well for no reason.
They aren't forcing you to do anything at all. I totally sympathize with your preference to be there when strangers are in your home (I'm the same way), but your presence is not in any way required. It would indeed be considerate of your landlord to inform you of cancelled showings, but failing to do so isn't forcing you to do anything. You're following your own preferences all the way here.
It's not up to you to "let" your landlord show people around the property when you are or are not there-- you are legally required to allow it, and your landlord probably doesn't care if you're there or not. Asserting an obligation which does not exist onto your landlord seems likely to cause miscommunications at best.
- Legal obligations tend to follow legal mechanisms
The law is on your side with notice periods, but legal action can be a strong step. Even the threat of legal action, however justified, could poison your relationship. And, honestly, this may or may not be the best possible outcome-- you are legally entitled to sufficient notice, and you don't have to compromise on that. A mildly surly landlord might be something you can live with. A very hostile one may not.
But to actually compel your landlord to satisfy their legal requirements, you'll have to report the violation and go through whatever processes follow. There is no halfway. If you threaten legal action, the landlord doesn't change, but you don't follow through, then nothing will change for the duration of your lease. If you do follow through, then you're going to get what the law says you should: maybe nothing (with a fine from the landlord paid to the government), maybe something awkward (being able to leave without penalty before the lease is up), and maybe something less than ideal (a cash payment from the landlord, plus unlimited ill will for the rest of your tenancy).
In any event, do some research and understand what the law will actually do for you. There is likely to be a moderate amount of stuff to go through between being aware that your landlord is breaking the law and getting a result (let alone a result you might actually want).