using a prepayment meter to collect a debt of £138 is like using a hammer to open a carton of juice, complete overkill.
Scottish Power are being unreasonable here. First of all, you haven't explained how the debt occurred. Was this through estimated bills? If so, you may have a case for complaint.
The first thing you should do, is notify them immediately that you are disputing the proposed plan and you would like them to put any further measures on hold while a solution is worked out between yourselves.
Personally, my advice would be to obtain the chief executives office address (they have complaints teams for these people) and write, recorded delivery to them. If they still do not see reason (i.e. a sensible payment plan over a period of months), then you can always contact Energywatch/Consumer Focus, who will mediate with Scottish Power on your behalf and come to an agreement that is best for both parties.