With the £10, it was exclusive of BPF so, you could list items up to and including £10 before BPF was added.
I assume it's the same for the new £20 limit.
Do remember, items must still weigh less than 100g, be letter/LL size and be listed in certain categories.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)