How can I optimize this gaming PC build for the best performance under £1,000?

You appear to have roughly £100 available in the budget, so putting that toward a 16GB graphics card would probably give you a more meaningful upgrade than spending extra elsewhere.

A Ryzen 5 5600-based version could improve the overall balance of the build. It lets you use an ATX motherboard, a faster and less expensive 1TB SSD, a stronger GPU, a case with much better airflow, and a more dependable PSU. The Ryzen 5 5600 includes a stock air cooler, so you can also remove the aftermarket cooler and put that money toward the other components. Here’s an example revised list: https://uk.pcpartpicker.com/list/r4HkjG

The RX 9060 XT 16GB is worth considering. It costs more than the standard RTX 5060, but its pricing is competitive with 16GB versions of the RTX 5060 Ti and it should offer stronger value for gaming in this budget range.

The biggest issue is the case airflow, since it has very limited ventilation. I’d also replace the power supply because that model is rated poorly for reliability. Spending a little more on a better PSU is worthwhile. Depending on the games and resolution you’re targeting, a 16GB graphics card such as the RX 9060 XT could also be a better long-term choice than the RTX 5060. You could save money by using a Ryzen 5 5600 instead of the Ryzen 7 5700X, since the gaming performance difference is fairly small.

Another possible revision is to switch to the RX 9060 XT 16GB and adjust the rest of the parts around it. You may need to add case fans depending on which case you choose and how many fans are included.