I found a website
https://www.prefixlist.com/ that has a drop-down list of container prefixes in alphabetical order. It appears to be a US site but intended to have international coverage.
There are several companies with names or branding starting Q and J, but I don't recall seeing the containers in the UK.
A couple of other container websites:
A useful PDF explaining the container size / type codes (e.g. 45G1)
https://www.bic-code.org/wp-content/uploads/2018/01/SizeAndType-Table1-3.pdf (part of the main
https://www.bic-code.org/ website for "The International Register of Container Owner Codes.")
Container tracking website:
https://www.track-trace.com/container
Enter a container number and it links through to various shipping websites for tracking information where available. Some companies (such as Maersk) allow you to enter a container number and see where it came from - for example I've entered a container number visible in a photo I've taken of a train, and you can see which port it sailed from to reach the UK, when it left and what vessel it was on.
The Marine Traffic website is also interesting for seeing where container ships are, with live maps:
https://www.marinetraffic.com