Here are a few random thoughts...
First: For a marathon WR to come off at London, factor #1 is something no one can control: the weather. The weather has to be excellent for anyone to take a shot at the WR. Too much wind, heat or rain and no world record is going to happen.
Second, Tadesse has only attempted one marathon and it resulted in a DNF. As another poster has noted, most people need several marathon attempts before getting an optimal performance from the marathon. With rare exceptions (e.g., Evans Rutto, Ondoro Osoro), most elites don't get their lifetime PR in their first race...in Tadesse's case, having never completed a marathon, asking for not only a great time but a WR is a bit much.
Third, fast half marathon times don't always translate to great marathon times. If this were a 30K race, it would be much easier to bet on Tadesse. But those last 6 miles of the marathon put it in a different category of racing. We've seen how good-but-not great half marathon times have turned into excellent performances in the marathon (e.g., Khalid Khannouchi, Sammy Wanjiru) and also how really fast half-marathon races have been followed up by poor marathon performances (e.g. James Kwambai). Superfast half-marathon times often don't translate into superfast marathon times.
Fourth, if a WR is going to be broken, Tadesse isn't the only one who will be going for it: Wanjiru, Kebede, Kibet, and possibly Kirui are capable of breaking it as well...and they'll be joined by several others in the lead pack. But as we've seen in some past London marathons, there are times when a field is so loaded that a race turns "tactical" (if one can call running sub 5 minute miles "tactical"). World Record attempts usually require some risk taking - after all, athletes are pushing the envelope to run faster than anyone has ever run in all of history. Sometimes, the risk pays off...and sometimes not. Elite runners can also hit the wall...so the athletes at London will probably have to make some decisions in the last half of the race about going for the record or going for the win.
All of which is to say that given good weather and a willingness to push the pace, the WR could go at London...but I'd be surprised if Tadesse were the one to break it.