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This year may well be different. Last year the new times were announced a week before vmlm, meaning London was the only real opportunity to achieve them and that turned out to be very hot where even champ level runners didn't manage gfa times.
You can normally get a pretty good idea by looking on runbritain rankings and seeing how many runners in the rankings have times between 2.45 and 3hrs within the qualifying window then going to v40 and see how many runners have a 3-3.05 time, then maybe add 750 on for the other cats or continue the exercise further down.