Mobile, while technically capable of this, does not fit the bill because their business model depends on locking people into long-term contracts or using it as a marketing platform (I've tried launching a startup that does away with all this, it's impossible).
Right now nobody will sell you a SIM with unlimited data (excluding Three because their "unlimited data" is so oversubscribed that it's unusable) or a large enough data cap no matter how much you are willing to pay. Your only option is to either sign up for a yearly contract (which involves wasting time with support if you ever need to make a change or need to increase your data cap) or use Pay & Go SIMs and top them up regularly which involves using an IVR and listening to minutes of upselling random bullshit - there's no API and the billing is intentionally complex (depositing money isn't enough - you have to use that balance to buy a "bundle" of data, and you can't stack these bundles meaning you need to do this every time it runs out).