You've got two (entirely separate) sets of legal rights:
The Consumer Contracts Regulations allow you to return goods that you've ordered online for a full refund. (There are a few exceptions but they're not relevant here). However the seller can insist that you pay the carriage costs when you return goods under those Regulations.
However, in respect of faulty goods, you're also covered by the Sale of Goods Act (for purchases made prior to 1 October this year) or the Consumer Rights Act (for purchases from 1 October onwards). Returning faulty goods under the relevant Act should not leave you out of pocket, therefore the trader must pay the carriage costs.
Contact the supplier (preferably by post, using Royal Mail's 'Signed For' service) to demand compensation for the carriage cost you've incurred. Make it clear that you're claiming under the Sale of Goods Act (or Consumer Rights Act, as appropriate), and not under the Consumer Contracts Regulations. State that, if you do not receive your money, legal action will follow.
If you still don't get compensation for your loss, claim online here:
https://www.moneyclaim.gov.uk/web/mcol/welcome