You may be confusing the UK pilots who obtained an ATPL having flown nothing more than a C150 for 1500 hours of which 100 was at night and JAR-FCL qualified ATPLs who are a totally different breed.
The only way to obtain an ATPL now is as follows;
Hold a CPL and a single pilot IR
Have completed an MCC course
Obtain a multi crew type rating and a multi crew IR
Amass a minimum total of 1500 hours with a minimum 100 hours night with 500 hours multi pilot and pass the required test.
If having at any stage one decides to do single pilot IFR flying privately, for the same price of doing the IMC renewal, one can do the single pilot IR test.
If the multi pilot IR is allowed to lapse then the pilot can not exercise the privileges of the ATPL or the multi pilot type rating included therein. They effectively become a CPL as far as privileges are concerned (assuming they have a valid Class 1 medical). If they only have a class 2 medical then they only have PPL privileges.
I don't know anyone who would spend the time and money to pass an IMC test when for the same time and money they could pass the single pilot IR test and not be restricted as per the IMC rating.
Regards,
DFC