Out of curiosity I looked at the price of getting from London to Gatwick by train. The cost was £16.60 single (two adults, one railcard), the same for a return. This surprised me, since we paid £8.30 for London to Brighton each way a couple of weeks ago.
Putting it in as one adult for a single, I got prices of £10 London/Gatwick and £5 for London/Brighton.
This just seems ridiculous. And I assume, since Advance tickets don't allow break of journey, I could neither leave nor board the train at Gatwick. Because the barriers wouldn't accept the ticket - not simply because I'd be in breach of contract.