I just did the calculations for someone and if you do the speed needed to reach the Mall before the half five cut off then you shouldn't get cut at any of the earlier ones either.
Not quite, the first cut off is quite aggressive for the late starters. It's 27 miles in, and you need to be there by 10:50 making it 1hr50 for 9am starters. That means you need to do the first 27 miles @ 15mph, whilst you only need to average 12mph to get around the whole course in 8.5 hours.
That said, if you can't do the first 27 miles @ 15mph, where it's fast and flat, you'd probably be slower than 12mph overall when you include the hills.
For anyone worried about their current speeds in training, you've obviously still got a bit of time to improve, but note that the closed roads will be worth around 1mph, and the adrenaline from the event will boost you another 1mph.