AFAIA the Consumer Rights Act 2015 covers your rights.
https://www.citizensadvice.org.uk/Global/Public/Campaigns/cra/ConsumerRightsSummary-GoodsOL.pdf
You have 14 days from the day after you receive your goods to reject them if you have bought on-line as you have not had a chance to inspect the goods before purchase. You can reject for any reason. If the item is not faulty and as described then you have to pay return carriage. If the item is faulty or not as described then the retailer has to pay for return costs. Failure to do so is an infringement of your statutory rights.
If you wish to return the bike because it is not as described the retailer must pay for the return. If they refuse and you paid by credit card then under s.75 of the consumer Credit Act you can enforce your statutory rights against your credit card provider. In practise this means commencing a dispute and a charge back for the amount you paid - forcible refund. Your credit card provider may also try to force you to return the bike at your cost. You don't have to, stick to your guns. The retailer should arrange collection at their expense.
If you paid by Paypal then I believe your credit card provider won't intervene as Paypal has it's own dispute and resolution process.
You need to work out what you want do.
Is the bike fine just showing signs of slight use? Negotiate a further discount or perhaps new tyres.