The UK has lots of regulatory bodies and they all work in broadly the same way. Provided you do the bare minimum to comply with the rules as defined in plain English by the regulator, you won't either be fined or personally liable. It's only companies that either repeatedly or maliciously fail to put basic measures in place that end up being prosecuted.

If someone starts maliciously uploading CSAM and reporting you, provided you can demonstrate you're taking whatever measures are recommended by Ofcom for the risk level of your business (e.g. deleting reported threads and reporting to police), you'll be absolutely fine. If anything, the regulators will likely prove to be quite toothless.