All very interesting, yet still inconclusive. I was actually on the phone to HMRC recently, and was told that their calculations for taxable interest are based on the information received from banks etc. So whether or not the interest is accessible becomes moot. Of course, the person I spoke to may have given me incorrect information, as it appears to contradict the 'guidance' given here by HMRC admin.
I'm sure this is why HMRC and my own figures are always at odds with each other, as I provide information based on whether interest is paid monthly or annually (i.e. when it is paid, even though I can't access it if paid monthly), yet banks' statements of interest often straddle tax years. So I may have received £1200 interest over a year, but if the 12 month fixed account was opened in October, the banks would only have 'paid' 6 months interest within the tax year. And that is the information received by HMRC.
In summary, I'm inclined to go along with others who have said they will provide HMRC with the information that allies with what their banks have provided, ignoring the whole accessibility issue. But that is not what I've done in the past!