There are several companies that provide a mobile service to offices, some static ones, and some that do both.
I can't remember any names at present, so I guess it's a web search for you.
Personally, I like to see that my stuff is actually shredded in front of me. I'm not at ease with people carting it off and saying they are going to do it.
Over a period of time, a home shredder may well be cost effective, as their prices have dropped. Use a cross-cut one, not just strips, and try to be as narrow as possible.
Note that Wirral Council do not accept shredded paper for recycling, so it has to go in general waste, but can be used in a composter, and paper takes up far more space when shredded.
Perhaps we should all make paper-mache 'bricks' from our shredded paper, and put them in the grey bin, until they can appoint a contactor who can deal with shredded paper.