One advantage (for me) was that if the Dunkirk evacuation had not been a success I might not be here; my grandfather was with the BEF in France in 1940.
Downside was that France fell: in the short term this was very bad for the British (in the long term, it got the Americans to give support quicker than had it might have otherwise if a WW1-type stalemate had occurred).
It would have been a great problem had 200,000+ UK troops become POWs; not insurmountable, even though it was the bulk of the experienced Regular Army, for conscription was kicking in and in any case they had to leave behind all their heavy equipment, but very bad for morale and might have prompted politicians to accept peace in order to have them returned. (It would also have left a significant hole in British manpower later on in the War, when they were also fighting Japan and garrisoning conquered nations.)
It appeared to be treated as a triumph; rightly so given the opposition they faced and the bleak outlook at the time, although in no way can it be construed as a genuine victory. One could say it provided a jolt to the Army to improve its efficiency before going on to campaign elsewhere.
Not heard much about this with regard to "pommie bashing" - be interested to hear examples of this. "French bashing" with regard to their generals in 1940 - mais oui! That is commonplace in most accounts.
To an extent it is an early Tobruk or Singapore, but with a "happier" ending in that most of the troops were saved.
Cheers
Richard
(Always pleased to help a Kiwi.
Are you sure you're not South African?)