Uk's Working Holiday Visa

i need info on applying for a working hol. visa in the uk.
i have checked the uk website but i want to know if they grant nigerians such visa and how often or easily one can get it .

i am a regular traveller  to the uk and i qualify to apply, i have a 2yrs visit visa and i would like to know if i can change it to a working hol. visa.

please, any one who has applied before should assist me on what to do and advise accordingly.