In an unregulated (= no slot) environment, a Tower's 'flexibility' may be storing up problems for sectors hundreds of miles away.
En-route capacity is finite, and the decision whether to regulate is based upon flight plan and radar data. If the flight plans do not indicate an excessive demand for a certain period (say 30 mins, but more often in the UK 15 mins) at a certain 'choke point', then no regulation will be applied.
Now let's assume you call for start 30-40 mins early and your local tower approves it, as there is no slot restriction. You get airborne, and arrive at the 'choke point' 30-40 mins earlier than filed. Now add several other airfields who have done the same (or let traffic depart 30-40 mins after EOBT) and you can see how a bunch of traffic can build up. Bunching of unregulated traffic is one of the biggest causes of overloads in ATC, and it is a serious safety risk.
If you are ready 40 mins early, you should re-file without relying upon the tower's flexibility. The rules which say you should for taxi no more than 15 mins before and no later than 15 mins after EOBT are there for a good reason. Please help by complying with them.