A simple rule works for most self-publishers: you usually need one ISBN for each format and edition of your book.
If you publish the same book as paperback, hardback and eBook, you will usually need separate ISBNs for each format.
That means one title can easily require two, three or more ISBNs depending on your publishing plan.
Retailers, distributors and libraries treat different formats as separate products. A paperback is not the same product as an eBook, and a revised edition is not the same product as the original edition.
Think ahead before publishing. If you expect to release print and digital editions, or multiple versions of the same title, it is better to plan for those ISBNs early.
If you only need a small number of official UK ISBNs, you can buy UK ISBNs here.