You can say in-ter-est, but it 's often shortened by reducing the middle syllable when spoken by natives. It's not just in British English, this also happens in the US.

In British English, it has 2 - in trest. In trest ing In trest ed

Three: in-ter-est The stressed syllable is 'in'

In British English it has 2 syllables: in - rest however most English speakers would say it has 3: In - ter - est

I'm a US native speaker, and in the US both pronunciations are common and normal. My personal favorite dictionary for US English, is the American Heritage Dictionary. I just checked, and it shows three pronunciations, and they are always listed in order of frequency: (ĭn´trĭst, -tər-ĭst, -trĕst′). So according to this dictionary, two syllables is more common than three... and there is variation in the vowel sound in -est. It might depend on context and euphony. I'm not sure, but if were saying "Please tell me some of your personal interests" I think I would pronounce "interests" as three syllables. If I were saying "The Fed raised the interest rate today" I would probably use two.