America, welcome to the party a little late. The 22-year-old British singer Natasha Bedingfield's ''These Words,'' included on Unwritten, has gone to No. 1 in a dozen countries and is just now hitting here. The song, a transcendent synth/R&B anthem, is undeniable; think a softer Pink at her peak, grrrly but less steely. Yes, it's pop, straight up: something you wouldn't be loath to buy your kid sister and wouldn't be embarrassed to borrow from her either.