McJiz,
As far as I know, the answers so far are correct - you must have the medical in your possession before you can use it. But regardless of the legal situation, you'd probably have trouble convincing anyone to hire an aircraft to you if you couldn't present your medical to them in any case.....
Good news that you've got your Class 1 - but you'll have to let the excitement build for a little longer!
FFF
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