If I told you that you couldn't start for 20 mins due airfield congestion, and you were ready for your eobt, then I wouldn't update the eobt, simply because you might end up getting a massive slot delay, and that wouldn't be fair on you.
You have (if I remeber rightly) 15mins from eobt to request start and then 30mins from eobt to get airborne, at the smaller airfields this shouldn't be a problem