At the end of every tax year you should set the dates for that whole tax year (e.g. 5 April 2014 to 5 April 2015). From this you can get an approximate tax liability for that period. (in this example a full tax year)
If there are unclassified transaction you cannot see due to them being unclassified you will get a warning message that there are transaction that could not be evaluated in terms of tax due to them being unclassified.
You can export the result to csv or xml (this can easily be imported into excel or other accounting software) and then combined with your other taxes by your accountant or tax professional.