I have not found an alternative, but I see that on their website under "Contact Us" they have a form for giving them feedback, so perhaps you could use this to advise them that you do not like/cannot use 0845 or 0870 numbers and ask them for a geographical alternative.  After all, they give the email address for Customer Service as:  whatdoyouthink@clearlybusiness.com
- so I would be inclined to take them at their word!
They may be one of these companies that is stuck in the past and think they are doing their customers a favour by providing an 0845 number.  On the other hand, I see they are owned by Barclays Bank, so perhaps I am being too charitable about their motives!