CB
If you have the hours requirement for ATPL issue you can do the skills test at any time.A far as I'm aware you can even hire a sim and pay a TRE to do the test for you if you wanted (and had cash to burn
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So I can't see why your LPC/LST at the end of your conversion course can't be your ATPL skills test as long as the TRE agrees. Just make sure he/she fills in the correct form as I know a few guys who have come unstuck that way!