I'm a common law barrister practising in London. I can help with this.
On the face of it, the statement is sales patter and not a representation that can give rise to a claim that the goods are not as described or faulty. Theoretically, you could argue the blankets are not fit for purpose but in reality I doubt this will work as it would be incredibly difficult to prove this (that the blankets are too hot in summer, for example).
Have you not paid for the blankets yet?
If you don't want the quilts, I recommend you return them and say you have rejected them. Say that they were sold as being fit for the purpose of use during the summer i.e. at ambient temperatures of above 18 degrees and that they are not fit for this purpose so they are either faulty or inherently not fit for purposes and therefore the contract had been breached.
They will disagree and demand payment but its doubtful they'll do anything about it. They would need to take you to court, which is expensive. To ward this off say you will instruct a bedding expert to provide a report to prove your counter-claim.
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