Where the commonly quoted figures come from
The Lexsure master wording states that most mortgage lenders require at least 85 years remaining, and separate wording records that an increasing number require 87. Both figures are real; neither is a rule of law, and neither is a survey of the whole market on the day you read this. They are what the professional wording records about lender practice at the time it was written.
Treat them as the shape of the answer rather than the answer. The useful question is not "is 85 the number?" but "what does the lender on this purchase require, and what will a buyer’s lender require when I sell?" — and those have documented answers for any particular lender.
The reason the figures cluster in the eighties is arithmetic rather than tradition, and it is worth understanding because it tells you how the number will move.