Using a borrowed cable will not invalidate any warranties. The Consumer Protection [Distance Selling]Regulations 2000 are an addition to other consumer laws. The good part about the Distance Selling Act is that you have a 7 day cooling off period, in which you can return a product for any reason, providing that it is in the same condition that you have received, including wrappings.
In your case now, you will be covered under the two other main consumer laws: The Consumer Protection Act 1987 [as amended] and The Supply of Goods and Services Act 1982[as amended].To learn more try click here or click here
If you are having problems now, think what you may have if a major problem arises. Consult the Trading Standards for further advice, you should not pay for Savastores errors.