Is 7 trains an hour to Leeds not enough to catch a train to London?
Irrelevant.
There are 5 fast trains per hour to Leeds, which are typically overcrowded, and as
MylesHSG says, no direct services so regardless of frequency it will be a faff for some people. There is an additional slow service typically operated by a Pacer.
To make it up to 7 trains you must be counting the via Bradford service which takes over an hour; the idea that you would use this to connect with a train to London is absurd. After less time than it takes to do York - London, you'd still be in Yorkshire!
The fastest journey opportunity is typically changing at York! Other journey opportunities include taking a Pacer (typically) to Wakefield or Leeds.
http://demo.fastjp.com/#fares?oorig=HUD&odest=KGX&odate=20170628&otime=1300&maxch=8&split=on
And on Monday 29th, trains between Leeds/Manchester and London at suitable times would have been full and standing, and of course the TPE services would have been wedged as usual.