There's always debate about what labels you can give yourself. Many argue (and I agree with them) is that the best labels are those bestowed on you by others. Nevertheless, some people choose to promote themselves to various degrees of hyperbole.
I know it's been mentioned here before, but the "Amazon best-selling author", or its shortened form "best-selling author" is still rife. This is trading on the fact that Amazon publishes hourly lists of sales performance in every category and sub-category (and even sub-sub-category) of interest. If you can persuade your pals to buy your books at a specific time, on a specific day, you can be ranked top of a list for an hour. Take a screenshot, and Hey Presto! - best-selling author! Except you aren't. It's what we call misrepresentation, especially if you drop the "Amazon"
Which brings me to "The UK's leading..." or "Europe's most sought-after..." If someone of respected stature has called you that, fine. If you made it up, it's probably not true.
So stick to the facts, please. Self-promotion is great. Over-claiming is inauthentic and dishonest.