No, you can't register to vote. It's completely normal to get these letters, they send them every so often to encourage those who can register to register. I got letters before I was eligible too. Also, some immigrants can vote in some elections (e.g. some Commonwealth citizens, European citizens), so just not being British isn't necessarily an impediment. In your case if you are American this doesn't apply to you and you can't vote at all, so just ignore the letters.