As mentioned by @pfm (thanks for the tag), these oversized cards have surfaces in both English and Italian. The Italian all came from a single eBay seller and were all 1st edition. The English ones came from multiple sources, and were a mixture of 1st edition; unlimited; and Base Set 2.
And Japanese ones have also been made in the past, as can be seen in the background of this picture (source: Japanese Base Set Jumbo Cards - Another Pokemon Mystery):
These oversized cards were usually used as either store displays (as in the picture above), or at events (like your London Toy Fair cards). I’ve also seen even larger examples (up to 2 meters high), which were used as a background for photographs. Here some examples of those which I’ve saved pictures of:
Out of curiosity: how thick are those cards you have? Rusty never answered my question back in 2019 when I look at the thread Scott linked. Based on your second picture, those English cards do seem to be thicker than the Italian ones I have.
Also, if you say they are 18 inch (which is ~45.7 cm), they’re apparently slightly smaller than my Italian ones, which have a height of 56.1 cm (~22 inch). I’ve always just assumed the English ones in Rusty’s videos were of similar dimensions as the Italian ones I own.
Also, as a Pikachu collector: if you ever decide to sell the Pikachu, lmk. 
And if you or anyone else ever gain more information about these, I’d love to know as well. (And I assume there was only one Pikachu among the 12 cards? Or were there more than one like the Charmander?)
Either way, would you mind taking a picture of the Pikachu next to a regular sized card?
Greetz,
Quuador