The British would have been constrained by the Washington Naval Treaty just as they originally were. From the RN point of view they had 7 carriers on their allowed tonnage just before the war started with several more in the works to replace some of the older units like Hermes and Eagle.
I really doubt that the RN given its monitary troubles would have made alot had the Germans built say, 3 or 4 carriers prior to 1939. The British top brass and politicians would have simply pointed to their greater numbers (albeit many of those carriers were of questionable value) and left it at that.

Carriers of the day also are cheaper than battleships, construct faster (18 to 24 months versus 24 to 36 for a battleship using European yard times), and consume less resources to build. The air wing is an added expense but, given the high rate of loss of aircraft operationally during this period (most had less than 100 hours flying time before being written off) this is not unduely high compared to land based aircraft.

In the Atlantic a German based carrier task group operating with just 2 carriers, a battlecruiser or two, several cruisers, and a dozen or so DD along with replenishment ships could have swept a 300 mile wide, 1000 mile long swath of ocean clean of shipping in a matter of weeks. The USN literally in the first 90 days of 1944 sank everything bigger than a canoe the Japanese had in the mid-Pacific in the way of shipping using their fast carriers. In just those 90 days the carriers matched the previous two years of submarine sinkings in terms of tonnage.

The RN would have been in a position where they had parity or were inferior in capacity in the face of such an operation. Their preponderance in battleships and cruisers would buy them little. Their carrier forces could counter the Germans but given the smaller air wings and far inferior aircraft they were using in the early days of the war the Germans would likely have shot their airwings to pieces for little return. The vaulted swordfish might have been a wonder against nothing more than antiaircraft fire, but faced with Me 109s or He 112s they would have looked more like Torpedo 8 at Midway.

This is one of those few 'what-if's' that really does give the Germans a means to change the outcome of the war dramatically. Thankfully for the Allies, the top German naval brass were the most hide-bound, traditional, behind the times bunch you could hope for.