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If you have been OCTA, and climb to cruise at the lower limit of CTA you are deemed to be OTCA. Many students have said that it should be called the upper limit of uncontrolled airspace, rather than the lower limit of controlled airspace. I must admit I agree.
If you are in CTA, you should remain 500ft above the lower limit to provide a vertical buffer with aircraft operating OCTA. AIP GEN 1.1 para 2.2.12. Once again, the wording of this paragraph does not imply that you MUST remain 500ft above the lower limit. I wonder who writes this stuff?
The AIP says that if you wish to have the protection of controlled airspace you should maintain 500ft separation from the lower limit.
However, this particular question specifies that you intend maintain 1000ft separation. Therefore, you should do what the question says.