I, at the very least once a week, often check Amazon UK to see what's new and coming up with Tolkien releases.

Today, I saw a few that were JUST added:

Letters from Father Christmas Centenary Edition Hardcover – 6 Oct 2020
Hardcover: 208 pages
Publisher: Mariner Books (6 Oct. 2020)
Language: English
ISBN-10: 0358389887
ISBN-13: 978035838988
https://www.amazon.co.uk/dp/0358389887/ref=sr_1_5?keywords=tolkien&qid=1574787994&s=books&sr=1-5

The History of Middle-Earth Part One Hardcover – 6 Oct 2020
Hardcover: 1920 pages
Publisher: Mariner Books (6 Oct. 2020)
Language: English
ISBN-10: 0358381711
ISBN-13: 9780358381716
https://www.amazon.co.uk/dp/0358381711/ref=sr_1_8?keywords=tolkien&qid=1574787994&s=books&sr=1-8

The History of Middle-Earth Part Two Hardcover – 6 Oct 2020
Hardcover: 2032 pages
Publisher: Mariner Books (6 Oct. 2020)
Language: English
ISBN-10: 035838172X
ISBN-13: 9780358381723
https://www.amazon.co.uk/dp/035838172X/ref=sr_1_7?keywords=tolkien&qid=1574787994&s=books&sr=1-7

The History of Middle-Earth Part Three Hardcover – 6 Oct 2020
Hardcover: 1488 pages
Publisher: Mariner Books (6 Oct. 2020)
Language: English
ISBN-10: 0358381738
ISBN-13: 9780358381730
https://www.amazon.co.uk/dp/0358381738/ref=sr_1_6?keywords=tolkien&qid=1574787994&s=books&sr=1-6

After visiting each page, it seems to me that these are US editions.Because of that, you may find better luck with American sites (Barnes & Noble, Amazon.COM...). Just hold onto those ISBN 13 #s.
Let's see what's different about them, or if they are 'mirrors' of HarperCollins' titles.