I have beem to United and West Brom on Virgin this season and haven't met an inspector once, United we walked throught the gates at Euston and the again at Manchester, same on the way back!
I hope for your sake you have booked seats! They always over book the Virgin trains.
If you can get a lift why risk it?
I would suggest you will be fine, if you download the national rail app you can check live trains and this will show you if you have any issues before leaving, so you will know to get yourself to Brighton.
Train leaving littlehampton and you should be fine...