I like the way that the GAR form works. But its easy to make a mistake. One is most likely to send GAR form at the same time as sending the outbound flightplan. However the important data concerns the UK return flight. This is likely (but not necessarily) on a different date than the outbound flight and it will definitely be at a different time. It could also very well be from a different airfield than the one you landed at overseas.

May I suggest as follows. If the route is outbound to an overseas airfield then and you click on the GAR button, then before the GAR form comes up, 4 bits of info are checked / requested.
1. Date of return to UK
2. Expected time of arrival back in UK.
3. Departure airfield for return journey
4. Arrival airfield in UK

This prevents the default  OUTBOUND departure/arrival airfields and the date/ timings for same being put into the GAR form.

If this is not possible or difficult then leave all these blank on the GAR form. I realise of course that the best approach is to have an entirely different FPL for the return journey and in any case one will have to file a return FPL.

Just a thought having nearly made such schoolboy errors for trip later this week.

Cheers
David