I'd like to thank Twitter user @verjoc for helping me get back to iOS 10.3.2 after 24 hours of being bricked and then stuck on iOS 11 beta 2!
I thought I would share the steps, as all seemed lost but there is a way to downgrade!
Make sure you've turned off Find My iPhone and your passcode before taking these steps.
1) Download iOS 10.3.3 Beta 4 >HERE< make sure you select the correct model of device you have.
2) Put your device into DFU mode. It varies between different models how to do this, so simply type into Google your iPhone make followed by 'DFU' mode. Once you know the correct buttons that need pressing, first open iTunes and plug your device into your computer and then follow the steps to get into DFU mode. The final outcome should show the iPhone screen black and a notification on iTunes to say the phone needs to restore.
3) Shift click 'Restore' on iTunes and select the iOS 10.3.3 Beta 4 IPSW file you just downloaded. Confirm you want to install this. Wait for this to install.
4) If you would prefer just to get back onto an official release of iOS, download iOS 10.3.2 >HERE< and follow step 2 and 3 again but choosing the iOS 10.3.2 file in step 3.
All done, and back to iOS 10! Yay!