There is a difference between strikings on a wrong flan that inadvertently got into the blank bucket and another on a properly sized size blank but an off-metal strike, and more so if there are any records. If the flan fits the dies then it is much more likely to be intentional.
The problem with wrong sized flans is that they are totally random and can be worth all or nothing on the day. It is also helpful if the mint was not striking many coins for third parties at the time, which I suspect would apply in your case. Post-war there were a lot of contracts with foreign governments and a noticeable increase in the number of wrong flan strikings, but prior to WW2 a lot of empire coins were done by Heatons, so RM off metal strikes are considerably rarer. The visual impact will invariably have an effect on desirability.
My example of a wrong flan is a 1967 florin on a Burundi 10Fr piece which is slightly small and 2/3 of the correct weight for a florin. It cost £1.79 incl. P&P on Ebay about 10 years ago. Not much to look at and didn't break the bank.