Non-geographical numbers, hark back to the days before most of us had vast amounts of free national calls etc, where phoning numbers outside of your local area, cost an arm and a leg, at which time, 08** numbers, were very handy. At that time, given the choice between calling a company in Stoke-On-Trent at national rates, was a lot less appealing than using a "local" rate number.
These days of course, its a lot different, but still most people (I say most, not all) would rather call an 08** number, than call direct, as they perceive an 08** number as being more professional.
As to why OCUK have different numbers for different parts of the company.. simple.. they're different parts of the company, just as my parents used to have two phone numbers for the house in which they lived, one for the 'family' and one for the 'business'.
Or would you rather press 1 for Sales, 2 for Returns, 3 to speak to an advisor, 4 for more options, 5 for less options, 6 to hear these options again, 7 to be put on hold for an hour?