I've no idea how long the Soda Stream will last .... You'll need to compare your existing co2 capacity/ usage time to that of the Soda Stream ... they normally contain 250 grammes of liquid co2.
The adaptor should allow you to simply screw on your existing regulator.... Australia has the same...
I don't know of any U.K. suppliers of adaptors...but here's an Aussie supplier.
http://craftbrewer.com.au/index.php?keywor...age=shop.browse
Regards Woody