Rachel, your safety trumps everything else, so you can abort any run (or interval or whatever) if you have any doubts over being able to run safely on frosty streets in the dark. If you can get on a treddie instead then great, but your marathon won't be wrecked if you miss a session or two to bad weather. It might be wrecked if you fall over and crack a rib. Trust me, you don't want to find out how hard you hit the ground if you fall running.
Lots of people seem to use porridge. Not my thing, but it seems to do the trick for others. Good point Will about using what they have on the race - VLM does have lucozade as you say,and water in bottles which are easier to drink from than cups. I don't know what they have at Manchester Rachel, it would be worth finding out.
Another reason to do a half is that it gives you a chance to try your nutrition in a race. It's never quite the same in a long run for some reason.,