0891 is one of the old premium rate codes. As far as I know, these were replaced some time back with 09 numbers to make the customer more aware of the charging band numbers are in.
01, 02, 03 - geo numbers (or for 03, non-geo charged the same as geo)
04 - was pagering, unsure as to use now
05 - VoIP and some old 0500 mercury numbers
06 - no marked usage
07 - mobile and "personal" numbering
08 - free phone, national services
09 - premium rate
To the OP, you can request out going call barring from BT. I seem to recall that you can have a permanent bar for certain types of numbers and also a PIN system that you can bar everything and then only unlock when you need to use the phone.
Freephone, operator and 999 are never barred.
Matt