I used the GAR filing for the first time last week when filing for a flight back from Caen to Bristol. The file plan filed fine and sent me a confirming e mail. The GAR seemed to file and gave me a reference number which i duly noted. Before departure, immigration at Bristol phoned me to say that the had spotted our intended arrival via the flight plan, but had received no GAR. I told them I had filed it 30 minutes before. They were not au fait with SkyDemon and asked where i and sent it to and of course I had no idea where I had sent it to. The lady searched but could find no GAR and phoned back, but we were airborne. On landing, they requested I resent the GAR which i again did on SD and again received no confirming e mail. The lady in immigration appreciated that we had done our best and requested that in future, we  copy her in with an e mail, which we could do, attaching either the written details or a screen capture of the SD GAR form. When I last heard she was searching in Manchester central immigration for any sign of our GAR. Am I missing some important step here? I had a good wifi when i sent the GARs and had no problem receiving e mail confirmation of the flight plan. Help please

The email reply from onlineGAR with a reference number is all that you are required to have,see attached file.
However it seems that some of the border force people do not understand their system.
I had a similar scenario recently and spoke to John Murray /AOPA about it.
Here is his edited reply:

***********
Pete
can you let me know 'who' said they don't get onlinegar
it is absolutely untrue - there are CBP terminals in every SB and BF office
I spoke with ***** SB...
They say they are 'encouraging' all to use the onlineGAR

- file online
- get your GAR ID
-  tell anyone who wants to know it
-and you should be fireproof

Seems to me the main  problem is ********** who have not read the Home Office Guidance to airfields which has just been reissued

I quote from it (same rules apply to SB and CTA flight) CBP is the backend to  OnlineGAR - ..................
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So as I understand it, they need to check their CBP terminal for your reference, where apparently they WILL find it!
regards
Pete