This very issue has been raised very recently. You haven't said but from your video you purchased the R7229.
This controller and the earlier R8250 controller are designed for modern low power low weight locomotives. They only have a maximum of 500mA of current output.
Given the age of your locos, they would benefit from a controller with a 1 amp output as the motors in the locos draw far more current than the modern locos.
The outputs of the R7229 / R8250 controllers are not pure DC, they output a technology called PWM (Pulse Width Modulation). The PWM in these cheap Hornby controllers operate at a low frequency and indeed do make some locos buzz when starting off and needing the knob turned up to get enough current flowing to move these old motors.
So the bottom line is that what you have observed is totally 100% normal for these particular PWM controllers when used with aged locos. Undesirable, but normal to expect. Somewhat borne out and confirmed in your video as the battery outputs pure DC and not PWM.
What is PWM? ... click this link and look at the fifth post down.
Note: I called the R7229 / R8250 cheap controllers. Some might consider that about £40 is not cheap. But this is a half to a third the price of a quality controller. These controllers were designed to slip into a 'Train Set' box just to give something to use by a child receiving a 'Set' as a present. They are not meant [in my opinion] to be serious layout controllers.
Further posts on this topic thread made by others will probably promote looking for H&M Clippers and Duettes on the eBay second-hand market. This is the solution that most members post when the same question as yours gets asked.
TIP: As a newbie poster on the forum, just be aware that the 'Blue Button with the White Arrow' is not a 'Reply to this post' button. If you want to reply to any of the posts, scroll down and write your reply in the reply text box at the bottom of the page and click the Green 'Reply' button.
See also – further TIPs on how to get the best user experience from this forum.