Philip
Donor
Historically it was England's last continental holding, so there's historical justification there.
Except it wasn't. Charles II sold Dunkirk to France 100 years after Calais fell. And that completely ignores that Hanover was held in personal union with the UK by George III.
It would give Britain a foothold on the continent too. It probably wouldn't really be worth it's cost in the long run, but there is a prestige factor too, and rulers and diplomats don't always act perfectly logically.
Again, Britain's foothold is Hanover. Hanover was far more prestigious than Calais before the war as it held electoral vote in the HRE. The thing is, GIII didn't seem to care. I fail to see why he would become interested in Calais now.