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NSK, so, how to be sure?
Your stamp is an extremely pale colour. Even allowing for distortion of scanning/photagraving and monitors, such a pale colour is a huge red flag.
If you look at the colour within the marginal scrolling on the left, you see traces of a much deeper colour. The stamp looks even paler around the effigy of the Queen. This might be because the cleaning concentrated on that area.
An issue with these stamps is that the watermark, regularly, shows through and appears like a cleaned cancellation. If you look at the Queen's cheeks and neck, and around her eye, there are backish smudges. This might be the watermark, but the blackish smudges do appear like blobs (removed cancel) and not like outlines (watermark). You might want to have a look from the back to determine whether these smudges align with the watermark, or not.
The lack of gum, although not irrefutable evidence of it having been used is another red flag.