Hello I am a lawyer with over 15 years experience I will try to help you with this.
There are different ways to become a British citizen. The most common is ‘naturalisation’ and this appears to be the right method for you.
You can apply for British citizenship by naturalisation if:
-you’re 18 or over
-you’re of sound mind
-you’re of good character
-you’ll continue to live in the UK
-you have met the knowledge of English and life in the UK requirements
You must usually also have:
-lived in the UK for at least the 5 years before the date of your application
-spent no more than 450 days outside the UK during those 5 years
-spent no more than 90 days outside the UK in the last 12 months
-been granted indefinite leave to stay in the UK, ie there’s no specific date that you have to leave
-had indefinite leave to stay in the UK for the last 12 months
-not broken any immigration laws while in the UK
If you follow this link you will go to the government guidance booklet on applying for naturalisation.
This second link will take you to the guidance to the booklet https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/285863/guide_an.pdf.
This is the link to the form.
From what you say you sound like you will qualify for naturalisation as a British citizen assuming you are of sound mind, good character and any absences are within the rules (see the booklet for details) You should fill in the form and send it with the correct fee (this link for a table of fees https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/261456/nationality-fees.pdf) to
UK Border Agency
New Hall Place
I hope this answer is helpful but if you have any further questions please feel free to ask.