I went BR on the Monday, my name was on the Insolvency Register on the Tuesday and in the Gazette on the Friday (may have been sooner but only checked on the Friday) so it is pretty quick.
I am not too sure when the bank accounts are frozen. I could login online to mine 1 week after my details appeared on the Register and in the Gazette but I haven't bothered accessing the account any other way as my bank is/was one of my creditors.