So frustrating! Is this rental a trap that must be avoided?

I want to rent an apartment. I've been searching online for the past few days, but many official websites don't have any available units. I initially didn't want to go through an agent, but due to desperation and not being able to find any listings, I contacted one. I paid a deposit of £150 per week and finally felt relieved. However, today the agent contacted me and said that the apartment has been booked by someone else. I said, "Then please refund my deposit." To my surprise, he said, "I can only refund £100 per week because he provided me with the information about the available units, and I gained knowledge from that information..." What is this? After a long argument and even bringing up the law, he finally refunded the entire deposit. Everyone really needs to be more cautious in these situations!