First Supercross - Solo Traveller UK - ADVICE?

I’d recommend booking a hotel that’s walking distance to Angel Stadium so you don’t have to deal with the whole parking situation and back up at the end of the race. 
Check if you can get in the pits for free by recycling a Monster Energy can, that’ll save you the pit passes. 
Plan ahead by knowing what time the A practice will be so you aren’t walking around the pits while the A guys are qualifying. 

Other than the $16 beers, you’re gonna have a blast !

(You should be able to sneak in some whiskey to mix with the stadium Cola, you’ll save some cash this route)

Anaheim itself is not a very interesting place (unless you like disney land) so other than the race it might not be the best place to spend your time here.

Any where along the coast is a good time... Venice Beach, Huntington Beach, Newport Beach, Laguna Beach. They all have their own vibe with good places to eat, shop and people watch. 

The traffic going in and out of the stadium really isn't that bad... for some odd reason, Americans have some insane aversion to sitting in traffic after any type of event. They will spend hundreds, sometimes thousands of dollars going to an event and then leave before the final whistle to save 10 minutes sitting in traffic.
Funny enough, I was listening to a podcast recently with Gary Neville and David Beckham, and this very thing came up. David said that his American friends never wanted to go watch any basketball games here in the states with him because he liked staying until the very end.

I was once in your shoes (living the UK and attending my first SX), and I can tell you that I had no desire to leave early. After the final race I stuck around and went back into the pits to see the bikes and riders one last time. Traffic wasn't a factor by the time I left. 

I’d elect to not stay anywhere near LA but that might be your thing. 
I’d stay in Huntington Beach near Main St or Newport near the pier for walkable nightlife. shortish drive to the stadium and back (I would assume you are going to the SX all day so parking outside the stadium grounds for free wouldn’t be an issue). You’ll have traffic getting out but it moves fairly quick and you’d be back to the beach to close the bars down. On Friday you could hit Pro Circuit (little inside museum of sorts) and Troy Lee’s shop just around the corner. You’d want to do this between 10 am and 2 pm to best avoid the freeway parking lot called “the 91”. There are other shops and tracks but that’ll depend on how much driving you want to do.  If you want to explore things outside of moto just google around as you can find just about anything fairly close from feet in the sand of the Pacific Ocean to feet in the snow in about a 2 hr trip.  Keep us posted. 

as an orange county resident for a decade, x100 everything here.

Myself and the crew have always used these bags to carry Whiskey in. Ain't getting screwed by that (expensive) watered down piss they sell.

Go to In and Out Burger

Go in when the gates open and catch all of practice.