How much is beer and food generally?(and tabs!)
I can't comment on the beer, and I don't think tabs are a thing, but food prices vary from stall to stall. Expect to pay between £5 and £10 for most stuff.
What to expect from the main arena and campgrounds?
Not sure what you're asking here, but here's the
site map for last year.
What sort of distance is stuff away from each other?
Everything is walkable.
Here's the googlemaps location for the festival site if you want to measure distances (note that the site is host to multiple events throughout the year, and the one that happened to be on at the time google last photoed it ISN'T Bloodstock, but does set up the site in a similar fashion).
Should i pack with the idea that I'll be well searched?
Yes. The queues to get in can get apocalyptic because of it. It does vary of course, some people may get a more thorough search than others.
Do any of the campgrounds have reputations?
Midgard is the original campsite and is also the biggest, so has gotten a bit of a reputation as the party camp. For better or worse, old school binjousting now seems to be a thing of the past, however.
The Ragnarok camp has a noise curfew, if you want a good nights sleep, that's your best bet.
Are they cool with portable BBQs?
Disposable ones only (unless you're in a campervan -- which you need to have a specific ticket for, and have to park in the campervan camp). Solid fuel camping stoves are also okay. No liquid fuel or gas canisters allowed on site (again, possible exception for campervans).
What's the weather generally like at BOA? (I know at download it's either death valley or a monsoon)
In my experience more often than not, it's dry if not sunny. It is during the British summer though, so there are the occasional downpours. FTR, last year was a mudbath. Checking the weather for the area closer to the festival and packing appropriately is advised.
How acccessible are the main routes around the site?(Do they have pop up metal roads)
No pop up roads (except at the coach drop-off). In terms of footpaths, when it's wet, it's a quagmire. When it's dry it's rock solid. There is no in-between.

Is there a supermarket on site?
Yep, in the Midgard campsite (alongside a camping shop), check the site map above.
Hope this helps, welcome to the forum (and festival).
"I don't always go to clubs... but when I do, I enter through the ladies room"
~ Adam Jenson