Rachel, have one between 5 & 6 miles, and see how you get on with that. Carry a second for emergencies but see if you can get to 12 without it. Ideally gels will be a bonus in races rather than essential so it's a good idea to try not to be too reliant on them. I've lost them in races before and you just have to get on with it.
Since we're talking half marathons, I've entered Wokingham next month. The first flat half I've done for ages so it'll be interesting to see how it goes.
Carter, I use compression socks in triathlons (well, the footless calf guards rather than socks), mainly to keep my calves in one piece during the run after all the cycling. They do seem to reduce the DOMS after those long races, but probably nothing that proper post-race massage wouldn't do. If you suffer with cramps they might be worth trying, but unless you've got problems I'd save your money for now.
Besides, they're not a great look on a runner...
...although I'm sure Rachel's are fine 