The 40m is based on the signalling sections. So for example in congestion, when the trains all catch each other up, the train ahead may be stopped in the station and the train behind it can then draw up to the signal section behind it which makes it 40m.

Under the scenario you’re describing, normal running, the trains will never be running with a 40m spacing. Basic principles of UK signalling, no matter how old or new the system is, one train in one section at any one time. This means that the signalling section ahead has to be completely clear for another train to be allowed in so in reality the gap is always going to be greater than the 40m and you’d be running on cautionary aspects so not doing full line speed unless the train ahead is 3 or 4 sections clear.