The UK annuals printed by Marvel Comics Ltd. are Eleven and a half by eight inches and originally sold for three pounds ninety-nine pence.
The annual also printed by Marvel Comic Ltd. titled "The Origin" is larger than the othersand features only one comic book, a reprint of the American Marvel comic ThunderCats #1 "Survival Run" and is eleven and a half inches by eight inches and a half. There was no "clip off" corner on the interior page with a price listed, it is listed on the back cover of the book as two pounds ninety-five pence.
I *do* for certain know that an American sized magazine bag (11 inches X 8 and a half- not including the fold over "tail" that you tape to seal the bag) and backing board (11 inches X 8 and a half) will fit all of the Marvel UK annuals, both sizes listed above, and I would wager a guess that they would also fit the larger individual UK comics as well. I cannot vouch for this, but I think it's a fairly safe bet.
I have the UK annuals pictured front covers, and the UK "The Origin" UK annual front and back to show the differences in the "'Roar's Thunderjunk" thread in the collector's showcase.
All of the UK annuals with the exception of "The Origin" have the same image both on the front and back.
Just as a side note, the American magazine plastic backers and bags I mentioned above also fit the Happy House ThunderCats coloring books as well.
I bought my bags and backers on e-Bay, this is not the brand I bought, but this is something you can maybe reference as you shop for protection for your collection.
https://www.ebay.com/itm/100-BCW-THICK-MAGAZINE-2-MIL-ARCHIVAL-POLY-BAGS-100-ACID-FREE-BACKER-BOARDS-/323077788734?hash=item4b38efd43e