Hi Arthur, Rayna's right... UKBA use DX for sending the BRP I just got it today
Got a letter as well through royal mail about my BRP and enclosed passport etc. but it's not here yet ( I meant the passport) ... Or maybe later ( was searching in google about it that DX for BRP documents passed will be returned through royal mail ) Still unbelievable though too early.. I thought it will take longer for you to have the BRP if you're going to apply via post which we did...

Good start of the year!!!
Cheers x