You should ask the store for a refund while you are at it, if they fell apart after one wash. You are legally entitled to your money back in the Uk if the goods are not fit for the purpose. If they fell apart at the first wash, they fall into that category as long as you followed the correct procedures. If you want someone to help you, you can click on this link.
http://www.scotss.org.uk/scotss2.htm This is a free service to assist you with any claim you have against a trader. It operates in a similar way to the BBB, but is a government agency. They may also be able to help you with BMI. If the shop also refuses to refund your money, you can file a claim at the same Small Claims Court link I just gave you!!