Do you know what start line you'll be going off?
The UK Marathon Championship start is directly behind the elites (Blue start) but I suspect you may be going off one of the other starts. Probably most of the people you'll be wanting to run with will be off the UK Championship start. If you off the Green start you join with them after about a mile. If you're off the Red start you join at 3 miles. You should find plenty of good groups running at around your pace.
I never bother taking the lucozade drinks. They have lucozade gels at about half way and 21 miles. I generally take 3 of my own, then grab a couple more at the half-way point. Might be worthwhile trying the lucozade gels if you plan on using the ones on course.
There's a solid downhill in the 3rd mile, so it's usually a fair bit faster. Other than that, it's pretty much flat. My favourite part of the course is coming over Tower Bridge at 20km (iconic land mark and big crowds), I always need to be careful not to get carried away and run too fast here. Around 19-20 miles the area is a bit bland and deserted but from 22 miles to the finish there are big crowds along the river and up past Big Ben.
There are clocks every mile and at the 5km timing mats.
Hope you enjoy it.