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A common-format residence card is one issued by an EEA or EU country whose design matches the one shown here. The most important features to look for is the red-blue-red background pattern, and stars and a bull symbol on the left-hand side above the photo. (The background pattern on the right is a national design that differs between member countries).
If you have such a card and satisfy certain other conditions (leaving by air before two midnights have passed etc.) then you should qualify for the landside Transit Without Visa scheme.