Hi Veloxuk,
What you describing is a perfectly valid business principle i'm afraid and is more-or-less the same throughout the energy market.
Pre-payment meters provide the lowest risk to your supplier - its a guarentee of payment.That said pre-payment meters have a safety mechanism built into them to provide emergency credit should you be unable to "top-up", so the chances of you being without power as you fear are generally quite low, it is also possible for extra credit to be provided by your supplier via an emergency credit, although these will obviously need to be paid back through your meter with your top-ups. Additionally where "force majure" conditions allow (this is where unusual conditions such as hurricanes, flooding etc.) most pre-payment meters are able to be set to temporary-credit meter operation. So in general terms unles something compeltley unpredicted occurs there isn't any reason you should be without power.
All Scottish Power are doing is replacing one security with another with their request for a security deposit. Given the current economic climate, the simple and unfortunate fact that more people are now unable/unwilling to apy for their energy consumption and the fact you've been a scottish power customer for 6 months (unless i'm misreading your opening post) - i'm sure if you were in a similar situation you'd like to protect your investment.
Additionaly if you are request work to be done (i.e. the exchange of the meter) should you not be responsible for payment of the work required? The same as if you modified your car, extended your house etc.
I know it's difficult considering how personal the situation but if you are able to stand back and look at the situation impartially you'll see all Scottish Power are doing is protecting their investment and protecting their costs - which as I mentioned before is perfectly natural and something everyone would want to do in a similar situation and climate.