Look a bigger and better business
We don't think anyone would argue that a business card or advert with a mobile phone number to call suggests that you are anything other than a 'one-man-band' business and sometimes this can send out the wrong signals to potential customers.
A local or national number says 'we are open for business' and also allows you to choose who you give your mobile number to. If the call rings on our free app, it is obvious that the call has come from your Virtual Landline and so you are prepared to answer in a more business-like manner.
Other small businesses choose multiple numbers from other towns in the local area to suggest a wider presence with other local depots. The same principle applies to companies or people outside the UK who wish to appear to have a UK office. What better than a London number for this?