Championship entries are only open to members of running clubs affiliated to British Athletics. I don't know whether not being a UK resident means you can't join a UK Club, but that might be an option.
I'm sure I've had this feeling of deja vu before...
Tom, as I said on the other thread, P&D is a plan plenty of people follow and there doesn't seem to be much wrong with it. It's your first one, so the basic task is to build up endurance - start running longer distances, but at the same time keep the pace down, slower than you're used to. Shorter stuff is where the speed comes in but you've already got more than enough for sub 3 and much nearer the time you'll need to think about matching speed & endurance to work out what your marathon pace should be.
Longer term you'll be looking at running a lot quicker than 2:59 if you've just run 1:15 for a half after 18 months, and are clearly still improving at that distance. I would say that 2:30 as a longer term marathon goal isn't unrealistic. But converting well from a half to full isn't easy after such a relatively short period of running - some can do it first time, most take a few years and several cycles of marathon training to get there.
Follow P&D (or similar) and you'll be fine. The hard part will be working out what your pace on the day should be, but it'll certainly be less than 3 hour pace.
Echo that on the win there Ryan, hard being out there on your own, definitely worth a few minutes.
Jools, I suspect you could run it without a watch and still know whether you were on 2:45 or 2:48 pace. I'm also confident that you're not going to let your eyes get too big and crash & burn. Way too sensible to do that.
Like the pacing strategy LS21. I like Chris Boardman's approach to pacing for the (cycling) hour record - he'd be constantly asking himself if he could keep that pace up to the end. If the answer was 'yes', it meant he was going too slowly, 'no' meant too fast - 'not sure' was the answer he was looking for. I've had that feeling in HMs recently when I'm hanging onto a group for a few miles, not sure if I'll be kicking on at 10 miles or drifting away. I don't think I'd try that approach in a marathon though, not until the last 10k at least!