UK Ancestry after a Holidaymaker?

Just wondering if anyone can provide advice on applying for an Ancestry visa after they travelled to the UK on a Working Holidaymaker visa? My girlfriend and I are going early September and there just seems to be too much red tape for me to get a UK Ancestry visa in time.

Actually it looks like my obtaining an A/V will void her application for a Holidaymaker since one of the conditions, located here, are:

"if married or in a civil partnership, your spouse or civil partner must also be eligible and you plan to take the working holiday together "

So my plan is to go over there on a holiday visa, and get the paper in motion for the UK Ancestry visa while I'm there. Can anyone think of anything in the criteria that would not allow me to do this?

I'll let someone else answer your "switching" question, but I wonder if you're misinterpreting the passage you've quoted from the BHC's website?
If you're not married to your girlfriend, and you're not in a formally-recognised civil partnership (which is exclusively for same-sex couples), I can't see how that paragraph relates to you....

Don't forget that time spent on a working holiday visa does not count towards indefinite leave to remain.

I know many (ignorant) people who had no idea they qualified for an ancestry visa, went over to the UK on a working holiday visa, and then only found out that they qualified for an ancestry visa when their working holiday visas were about to expire and they were looking for alternative ways to stay in the UK. Unfortunately they all wasted 2 years in the UK that could have potentially counted towards indefinite leave to remain.