In Canada 1500m is probably a short freight train on some routes - they can be more than twice that.
In the UK 750m is treated as a notional limit - thuogh there have been longer trains in special circumstances. In practice the maximum length is often lower because of line restriction (eg. lengths of passing loops) eg. 450m.
A major issue for the UK is that the long distance lines are generally dominated by passenger services. Passenger services tend to run faster than freight. Conversely the long distance lines in Canada are dominated by freight. This means that they don't have the problem of trying to mix these typically much slower long freight trains with fast passenger services.