Most people don't need to complete an annual tax return. Take a look here
for information on when you might need to complete one.
The £27,000 figure is the income level point at which you start to lose the extra personal allowance you get if you were born before 6 April 1948 or 6 April 1938 or 6 April 1935 (married couple's allowance). There is information here
on age allowances and how they work. Take a look here
for the allowance levels. So, assuming that all the income you do have which is taxable has been taxed at source or is completely covered or partially covered by your personal allowance and the total is £27,000 or less, then you won't need to complete an annual tax return.
If your personal allowance is absorbed completely by your pension income, then any bank interest you receive other then through ISA accounts or other tax exempt accounts should have basic rate tax deducted at source. Dividends from shares you own carry a 10% tax credit which is deemed to cover your basic rate tax liability.
You can ask the tax office at the end of the tax year to review your tax postion without necessarily having to complete a tax return.
I hope this helps but let me know if you have any further questions.