Hi, Nick,
Thanks for visiting the Apple Support Communities to get help with your iPhone camera. If we understand, the front FaceTime camera is not working with certain apps. We are happy to help!
If the issue is happening with all apps, including the native iOS Camera app, please refer to the recommended troubleshooting here: If the camera or flash on your iPhone, iPad, or iPod touch isn't working - Apple Support
If you find the issue occurs only in third-party apps, like the ones you mentioned (WhatsApp, Revolut), we suggest making sure those apps are up-to-date: How to update apps manually on your Apple device – Apple Support
Should you still need help after that, please get in touch with Apple Support directly: Get Support
Have a wonderful day!