Serbs weren't "forced" to do anything -- they were on the winning side of the war, when Croatia was on the losing side. The British could have cared less about what happened to Croatia post-war, they had their own worries. Besides, Dalmatia wasn't even part of Croatia until after WWII when Tito gave it to them.
I suggest you pick up a history book or two. When the Russians were knocked out of the war by the Bolsheviks, the main backers for the Treaty of London were gone.
On the island of Corfu, the British insisted upon Pasic to accept a state that reached towards Carinthia and Istria instead of the one promised in the Treaty of London.
Again, pick up a history book. Dalmatia was the centre of the Croatian kingdom....all the royal capitals of Croatia were in Dalmatia, all Croatia's kings were Dalmatian.
When the Serbs and Croatians came to a deal in 1939 (Cvetkovic-Macek Sporazum), the whole of Dalmatia was included within the Croatian Banovina.
Now let's see if you'll again engage in Serbian amnesia the next time this historical fact is brought up.