Landlord owns the house and sets the rent, simple as that. However, your tenancy agreement is a contract with him/her which should guide what each of you can do and when.
S/he can increase the rent when the contract comes up for renewal. When does your tenancy end?
At present, subletting allows you to save money in several ways. 1) it contributes towards your rent (nor the landlord’s. 2) It will reduce your council tax payment by 30% because you’ll go from paying all of the 75% rate to half of the 100% rate. 3) It will bring your bills down because they won’t double by having a roommate in with you, they might increase by 30-40% for example, which means you’ll pay less than you do now if they’re halved.
If you’re signed into a rolling contract with the option to renew annually then there will probably be a clause built in which sets how much the rent goes up by (usually it’s RPI or 3% whichever is higher, something like that).
If you’re not signed into a rolling contract (usually these have a three year term but the term is negotiable before you sign) or it has come to an end (eg 2 x 3 years), then the landlord can increase it as much as s/he wants because you’re not in a contract with them beyond the end of this tenancy year.
It may be that you are in breach of contract if you have sublet the property, even if the landlord has agreed that. This could also change things.
You need to speak to your managing agent for advice. But quite honestly the landlord has absolutely no moral obligation at all, they do however have a legal obligation and you’ve not said anything which suggests they are breaking that so far. Even though you’re likely to be in breach of contract by subletting.
If you can give more information on 1) how many times in the last 6 years has rent gone up, and by how much and 2) when does you tenancy year end and what is written in your contract about increases in the event it’s a rolling contract, that would help.
Sorry to hear you’re in this difficult position.