Thanks Daphne.
I think working holiday visa is a little bit different though, since:
"A working holiday visa differed from UK work permits .... in that its main purpose was not to bring immigrants to the country to fill job vacancies but to grant leave to enter the UK to those interested in working overseas."
http://www.Named OISC advisers.com/uk_immigrati ... _visa.html
So I guess someone on working holiday visa was not supposed to switch to work permit, hence the good advise from your lawyer.
Students however could switch to work permit.
So my question is still open: is it ok to have my empoyment start date on HR letter (which started while I was on student visa, 1.5 months BEFORE my work permit was issued) or shall I ask the HR to word it differently and mention my work permit issue date instead? Any suggestions for a wording? Thanks!