. Is it true utility rooms have to have an extractor fan fitted?

No

Part F of the Approved documents says otherwise.

http://www.planningportal.gov.uk/upl...F_ADF_2006.pdf

See table 1.1a on page 10 for minimum extract rates, and subsequent
sections for alternatives such as passive stack ventilation.

The only parts of a dwelling wholly exempted from a requirement for
ventilation are areas into which people do not normally go,areas which
are used solely for storage or a garage used solely in connection with
a single dwelling.