That site is good but I prefer this one which gives the whole years issues on a single page.

https://www.collectgbstamps.co.uk/explore/years/?year=1840

Just choose your year or stamp design.

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Mixed franking with stamp and Horizon (counter) label for a large letter up to 100 grams, World Zone 1, GBP 3.66. Looks correct to me. This is one of the few ways to obtain postally used UK stamps with post office date cancels. Ordinary letters franked only with stamps aren't postmarked any more in the post offices, but in the mail centres. But if part of the postage is paid over the counter, and a label is applied, any stamps are to be hand cancelled.

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