I don't believe any network guarantees instant delivery.
They don't, because they can't.
If you make a phone call and the phone signal is too weak for either party, or there are network problems, the phone call won't connect and is unsuccessful. This is because it is synchronous, i.e. has to happen at the same time.
If you send a text, as long as the sender has enough signal to make a weak connection, it will send the text. If the sender doesn't have enough connection it will either fail or say it will try again later. Once it's left the sender's phone, instant delivery still can't be guaranteed - there could be problems on the sender's network, or the receiver's network, or the receiver''s phone could be off or have a certain weak signal. They can't guarantee delivery, because they don't control all the elements involved in sending and receiving. Vodafone could send a text out, but they can't deliver it if the recover is on O2 and their network has gone down or because the receiver's phone is off.
The sender side will keep trying to deliver until it's succeeded, but after the first attempts right after sending, it'll try with longer gaps. I'm not sure off-hand at what point it decides to give up and say it failed - probably 12 or 24 hours or something. But until it has failed, it will keep trying, so you might get a text at 3am.
If late night texts disturb you, by all means tell your contacts who do it - but you need to recognise that it's not always down to the sender, so the bit you can control is blocking repeat offenders, and putting your text alerts on silent with the various methods available on different phones, or even turning your phone off overnight. If you think you can't make texts silent, then you can either learn how to do it or get a new handset which can or take the risk that it's going to happen sometimes, and it might not always be the fault of a human. But you cannot blame others for your own lack of understanding of the technology and how to use it to make sure you're only disturbed when it suits you. If I don't want to be disturbed, my phone is silent or off completely (except when I'm on-call, when I don't want to be disturbed, but accept it might happen.) If you can't do that, like many other things in life, you have to accept a compromise which means accepting you might sometimes get a text at anti-social hours, as well as a possible emergency call.