You make it sound as though completing an online tax return and talking to and getting advice from your local tax office are mutually exclusive. They are not.
Last year was the first time that 20P appeared and needed to be checked/not-checked… it was something new which the Tax Authorities had put into the system…
Not mutually exclusive… but Declaring on-line is/was supposed to reduce the contact with Tax folk and their staff numbers have been reduced accordingly (certainly in my Tax area).
I had never needed to ask the Tax lady for help when using on-line in the past… although I know her well from those occasions when I introduce newcomers… 
There have been moves afoot to close that Tax office altogether, but petitions have made them think again… for the moment. 
Exactly.
My point is that if you go into each tax new exercise with no preconceptions left over from the previous year, then whether the boxes are the same as last year or not, is completely irrelevant.
not sure how that relates to 20P… as I say, it caused a major hiccup… so not just me being soppy…
Stella is absolutely right regarding 2OP. We also made the same mistake and when we went to the tax office to get it corrected we were told that many people made the same faux pas. We shall still do it online this year though.