First of all, it is not an extension, it's an entry stamp if you enter on the Non-O multiple entry visa just before the "valid until" date.
Your entry stamp is no longer valid when you exit Thailand without a re-entry permit. No problem to apply for a new visa as your previous visa itself has already expired.
Just you know, London has no longer issued Non-O multiple entry visas, they issue only Single Entry visas for those.
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