I suppose you have already excluded the obvious blue and purple/violet cancels and non-private company perfins. Plus the postage issues that were used as revenues/fiscals.
You'll have to probably have most of a postal cancel on a stamp to prove it's postally used. some of the ones reading "stamp office" and "department" look close to postal types unless you have a reference collection.
For the decimal postal-fiscals, the revenue cancels give a region, postal cancels give a town or city/city office.
Why are parcel cancels considered undesirable?
Less desirable, I would say. It's because they are typically heavy and overwhelm the design. If it's not important to you, that's good because you'll be able to pay much less and fill spots that most others will not.
Postal usage of a lot of postal-fiscals particularly those over £1 can be extremely rare. Unless you have deep pockets for those, you may never even see them. And there's the usual warning about forged cancels on those.