When you buy something online, usually, somebody, somewhere, gets a commission for taking you to the site that you bought it from. Going direct to a site saves that business money as they have no need to pay any commission. There are sites that exist to help you find a shopping site and take you there, then when they get the commission they pass it back to you. So you can earn money back on your online shopping at thousands of UK retailers.
Before I buy ANYTHING (PC Stuff, takeaway, insurance, breakdown policies, whatever!) I check if there’s cashback available first. It might be 1% or 10%, it might be a fixed amount, it might not be much, but it might be well worth having, and even if it’s just a little, it all adds up!