I think you've likely become complacent over having had low rents for so many years, and instead of putting money by into savings for inevitable rent increases, you've just not factored it in.
Yes, it's a LLs responsibility to pay for insurance and maintenance, but these costs are factored into the rent they charge, it's not a feasible business if they do not factor in associated costs of running the property business. I found it a little odd and unreasonable that you don't seem to think associated running costs should impact the rent you pay. If it's a private rental, then it's a business, and if it isn't at least profitable, there's no point for a LL to take the financial risk of having such a business.
If there are numerous issues that you are unhappy with at the property, you and your LL is not addressing them, you can either move out and rent a property that is more suitable, or you can contact Environmental Health, if appropriate.
If the rent increase keeps your rent inline or below market rates, there's nothing you can do, other than move out. You will only be successful at Tribunal if it's above market rate and an unjustifiable increase.
Please also bare in mind that since the RRA come into force this year, LLs are getting clobbered, my letting agent now charges an extortionate amount to issue Section 13s, which are a very simple form, and used to be free to issue. Now, it's no longer financially viable for me to introduce small rent increases (used to be £10 - £25/mth, every 18mths), so now I'm either leaving the rent as is, or doing large increases of £100/mth (which I don't like to do as I feel it's too much).
Eitherway, Labour and Shelter have really fucked up the private rental sector. They want small independent LLs to sell up so that the huge banks can buy up the properties, this has been in the making for several years now and the impact of it is getting felt more each year.
This is the first year I've actually considered selling up...