Quick questions for any British train users here, i've had a quick browse about NRE's site, but it doesn't seem too specific.
Basically, i'm travelling from Huddersfield, up here in West Yorkshire, to Chessington South, down in Surry, via Manchester Piccadilly, Euston, Vauxhall, and my favorite part of it all, The Tube. Anyway, i've been on National Railway Enquries, tickets for my journey would be around £88 for a super off-peak return, or or £143 for an off-peak. But what may I ask is exactly the difference, one is super and very cheap, the other is ...not super. But what exactly are the differences, is there something incredibly taxing and extremely limiting I have to follow?
Basically, my plan is to be out of Huddersfield by about 12:00, leave Manchester at 13:00, be in Euston for around 15:20, arrive at Chessington about 16:50*, if the times matter. I always use the West Coast Mainline, thats of course Virgin (woo) which is alright, cause usually the tickets if they do say a limit they do say Manchester, i was wondering if a SUPER off-peak would have other things to make it so SUPER cheap.
Any help is very appreciated, i don't want to get screwed over, or stuck at Euston, or anything ridiculous

Note: Route isn't changable, I always do Huddersfield --> Manchester Piccadilly --> Euston --> Vauxhall --> Chessington South, i'm not changing it now, i'll get lost

* - I didn't actually check exact times, but they're the usual times i usually go around at, so times will be correct to within 30 minutes either way.