Eligibility of the working holidaymaker visa

[daYZman]

As I understand it, the working holidaymaker visa only allows the holder to work for at most 12 months out of the 24 months in the UK - hence, working in the UK isn't the "main purpose". However, if a holder plans to stay in the UK for 12 months and ends up working for 12 months, will he still be regarded as a working holidaymaker? It seems the 12 month allowance is permitted only if the applicant intends to stay for 24 months, but I can't find the calculation for stays less than that.

Also, what happens if an applicant claims he/she'll stay in the UK for 24 months (hence, he/she'd get the 12 month allowance) and ends up working for 12 months and leaves then? Thanks.