Comment by md_

From the Wikipedia article:

"There are also cases where the principle appears to have an effect, but weakly...In the United Kingdom, the SDP–Liberal Alliance, and later Liberal Democrats, between the February 1974 and 2015 elections obtained 1–10% of seats forming a third party, albeit with significantly fewer seats.[16] This share of seats is despite gathering around a fifth of votes consistently over the same time period.

In the UK there is no president and thus no unifying election to force party mergers and regional two-party systems are formed. This is because Duverger's law says that the number of viable parties is one plus the number of seats in a constituency."