The online retailer may argue that you diminished the value of the shoes beyond what was necessary to judge them because you wore the shoes outside and removed the tags.
The relevant law is the The Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013. https://www.legislation.gov.uk/uksi/2013/3134/contents
Right to cancel
29.—(1) The consumer may cancel a distance or off-premises contract at any time in the cancellation period without giving any reason, and
without incurring any liability except under these provisions— ...
(b)regulation 34(9) (where value of goods diminished by consumer
Reimbursement by trader in the event of withdrawal or cancellation
34.—(1) The trader must reimburse all payments, other than payments for delivery, received from the consumer, subject to paragraph (10).
(9) If (in the case of a sales contract) the value of the goods is
diminished by any amount as a result of handling of the goods by the
consumer beyond what is necessary to establish the nature,
characteristics and functioning of the goods, the trader may recover
that amount from the consumer, up to the contract price.
(10) An amount that may be recovered under paragraph (9)— (a)may be
deducted from the amount to be reimbursed under paragraph (1); ...
Imagine if you did have an absolute right to a total refund. You could have brand new clothes for no cost other than your time ordering and returning them. The retailer would bear the costs of buying the items in the first instance, processing your orders and returns, and restoring the returned items to a saleable condition.