You can apply for Indefinite Leave to remain on the basis of your 14 years residence in the UK. To get this you will simply need to demonstrate to the Home Office that you have been resident in the UK for the last 14 years without having been served with removal directions. In terms of documentation needed, a good rule of thumb if you can is 2 pieces per year at 6 month intervals but if this is not possible then as long as you have documents showing you were resident over the whole 14 years then that will be fine. If you have been working in the school since 1990 then your employer should be able to give you a letter confirming your length of employment. You must have been paying tax and so your P60s will suffice. Other documentation can be letters from your doctor confirming you are registered as a patient; utility bills; council tax statements etc. Any documentation that comes from an official source will be useful as long as it contains a date. SET(O) is the right application form for this.
There is no income requirement, it is all based on length of residence, good character and ties to the UK.
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