There’s no validation process, even for BBC iPlayer. They ask for a postcode because the regional content within the UK is locked to different countries and regions (ITVX content is on STV player in Scotland).

There isn’t a way for them to check your TV licence on ITVX. Even then a TV licence is only required if you watch the livestream broadcasts on the app. If you watch catch-up, streamed but not live broadcasts, a TV licence isn’t required except for BBC iPlayer. An IP address outside the UK isn’t likely to work either.