1942 Cover. Official Paid Cancel , "London Gt Britain"

Can anyone help me with some background on this meter? For a start, is it a meter or a cancel?!
For me, it was unusual to see "Great Britain" on the datestamp, but is it more common than I think?
Thanks for any info!

It is a Postmark / Cancel, and yes they are common as there were Millions of pieces of correspondence sent during the War.

I like that cover, a keeper.
Great example of war economy stationery.

An example with "economy label" to re use covers :

Thanks Stallzer for confirming. Title updated.

I was just wondering about the country name, "Gt Britain" being included on the date stamp, as I haven't seen that before.

Rod, I like the use of the economy label alongside the slogan cancel about wartime reuse of other commodities. Can you make out the whole slogan? Does that say "metals" after paper?

Fortunately Stampgal,
I'll collect anything and scan anything

Here is the slogan cancel from 1940
(notice the "ROM" on my example previous
does not belong to the slogan, what it is I have no idea)

Update: Ah! I see, it is a part further cancel from the economy
label that is ripped and folded out ...

"Save waste paper, Metals, Bones Rags"

Note also, the "glassine" window envelope is still clear
after 70 years.

Rod, my guess is your ROM is a part of "RECEIVED FROM" stamped on to the second (economy) label and spilling over onto the original envelope.

You are quicker than me Stampgal Thanks.

Stampgal, I wonder if the Gt Britain was only used on Official mail since it is in the spot of the CDS where the sending PO normally is. I have some Official Envelopes from that time period that I will check when I get home later tonight.