Consumers, for whatever reason often do feel reassured by a fixed line UK number, especially if it happens to be one whose exchange they recognise.
Would you trust a new bank, insurer, garage, doctor, cab company who only had a mobile number?
It is all subjective and probably not that rational to trust a fixed line over a mobile but if you are fighting for business, you use every tool at your disposal.
endorphin Alternatively why not buy and use an 0345 number instead?
According to BT, renting an 0800 or 0345 number, incurs in addition to the monthly fee a contribution to the incoming call costs and that is something that is difficult to cap or control.