Don't worry, the boxset includes all the episodes. There was no official Series 13 in the UK for superstitious reasons - the episode released as 'Series 13' in Region 1 is simply included under Series 12 in this item.
Series 13 was never named in the UK for superstitious reasons with the number 13. For that reason, the series 13 in the UK became part of Series 14. I can assure you that every single episode of Frost is on the complete Boxed Set series 1-15 along with subtitles. I do have that Boxed Set myself! Region 1, is of course America and Canada, and their listings are different.