It is linked to parental income - there is a minimum loan towards maintenance that every child is entitled to - here in Scotland, it is £4,500 per annum - but in our experience it costs nearer £8,500 per year for them to enjoy their little luxuries - eating and living indoors (depending on where they are studying - one is studying in Scotland, where the costs seem to be less, and the other is in England, and it is more expensive.
This is the site for student finance in England and this is the site for the Scottish Academic Awards service - you should be able to find the information you need there.
As far as I am aware, it is calculated on gross parental income, not disposable income - that is certainly the case here in Scotland.
Dh has devised a savings plan whereby we can set aside the money we need to give the boys, so each year's money is saved up before the academic year starts. They get their loans from the SAAS, and then he transfers over money from the savings each month.
Don't forget, your children could look for work once they are at university, to supplement their income from you and their loan.
On the English website, I have just been through the finance calculator, giving income of £50K, £100K and £150K (and assuming a full time course, living away from home and outside London), and for the last two, you get a maximum maintenance loan of £3610, and if your income is £50K, you get a maintenance loan of £4843 - so basically, you are not going to have to fund all of your dc's maintenance costs, but you are going to have to fund some. You will need to run this calculator with your actual household income and the correct details of the courses your dc are considering, to get a better idea of what costs you will be facing.
The tuition loan (for the fees) is NOT index linked - you don't have to worry about that one.
Hope this helps.