If Germany had not allowed the bulk of the regular British Army to escape, Britain would probably have been forced to capitulate.
Britain of course became an island fortress and later giant base for the bombing of German cities, for naval forces and airpower protecting the Atlanitc convoys routes and for the jumping off point for the invasion of North Africa and Europe.
With Britain still in the war and later America to join with Britain precious divisions were posted to defend the coasts from Norway to the Bay of Biscay and to protect the Mediterranean. There was less manpower available to hold back the Soviets after the failure of Barbarossa.
The defence establishment actually planned for being expelled from the Continent (thought not leaving all the heavy kit behind) whereupon Fighter Command would defend Britain while the army built up its strength.
In the interim, Bomber Command and the RN would engage the enemy wherever and whenever it could.
It would be a stretch to say Dunkirk was anticipated, but the idea that the army would be excluded from mainland Europe and would have to force a landing at some point was one eventuality that was actively addressed. Given that, I wouldn't say Dunkirk was quite the disaster it's sometimes made out to be.