Did British aircraft fly at a specific speed while returning from missions during WWII?
Ground speed
Real time wind components were not available to the pilots. They wouldn't have been able to tell the ground speed. Especially over the English Channel on a calm or foggy day where a drift meter is not very useful (if equipped).

[A drift meter] indicates the aircraft's drift angle due to winds aloft, and can be used to calculate the ground speed.

If they flew at an agreed true airspeed, then the controllers wouldn't have been able to tell that true airspeed.
Early IFF
Looking into the history of IFF and radar in WWII, it's apparent from early on radio IFF was used. It started with the pip-squeak system.
Mistaken identity
In the Battle of Barking Creek a radar issue caused a friendly fire incident where returning British fighters were attacked. According to Wikipedia, as of 2010 the final report has not been declassified yet.
Another one was the air attack on the fortress of Koepenick, the Germans thought there was an air raid en-route, they launched fighters, only to mistake their fighters for the raid, only to send more fighters, etc. And there was no raid, just the Americans were returning from a bombing mission and judging their direction of flight aurally led to this confusion. The German fighters started to run out of fuel and landed.

Göring freely admitted the laugh was on him, that he had sent the Luftwaffe on a mammoth tour of their own air space.

Navigation
Wind and ground speed aside, if we say it was a certain route combined with speed, that would require a level of area navigation that just wasn't there in WWII.
Logistics
Logistically, when an operation is launched, the air command has an idea of when to expect its return.
Speeds
Comparing the Focke-Wulf Fw 190 with the Supermarine Spitfire, the German Fw 190 was faster. 426 mph compared to 370 mph.
As for slow speeds, slow flying on the return leg is just calling for the surviving planes to be caught up with and shot down.

I'm calling it myth busted.