can i spend more than 90 days outside the UK when i am a resident?

i am a UK resident originating from Thailand. My family moved to the UK in the year 2002 and we have successfully applied to be a residence last year. Now as i have finished my second year at college, i am wanting to go back to Thailand for a year to volunteer and find work experience at advertising company. I planned out my whole trip and bought a one way ticket, but now somebody has just told me that i cannot leave the UK for more than 90 days. If I do go outside the uk for more than 90 days, when i come back from my gap year, i will not be able to apply for a british passport. This is stressing me out so much right now, my family has waited 8 years so we can become a resident and have a british passport. Can someone please help me? my mum wants me to come back after 85 days and wait for another 3 months to apply for a british passport but this will interfere so much with my plan. is there anyway that i can stay in thailand for more than 90 days? should i come back in 85 days and stay in england for a couple of days then leave to for thailand again?

sorry for the essay, any help would be so appreciated