''The UK SMF is set at 3.5NM or less and 600ft or less. The 600ft figure is based around the possible 400ft MODE C tolerance for levels I guess.''
In TC, where standard separation is 3 miles, the SMF is set at LESS THAN 2NM and/or 600ft or less.
As a former colleague wrote on his CA1261 (shortly before he retired) after cutting it a little fine on LL Approach:
2.00 miles = HERO
1.99 miles = W****R