The Malta's would have been a much better post war carrier than the Audacious class were, largly for the reasons that Riain has already stipulated. however, getting them in service would probably have required them to begin building far earlier, say 1943 at the latest to avoid them being totally cancelled and scrapped on the slips.
However, it should be noted that the constant comparison to the Midways is rather misleading. Instead, in terms of size (but not tonnage), crew and aircraft carrier they would be closer to that of the Essexes - roughly 900 foot long and between 40-50 aircraft. Think USS Oriskany.
As for general service, they would have helped avoid many of the major problems that plagued British carrier ops later on, such as an extensive redesign of the F-4 Phantom to operate off the smaller decks. Consequently they may give the British the edge to stay in the CTOL carrier business for longer, although this is more to do with economics and politics than actual operational experience. They will still also be of poor steel quality (as all wartime builds were) and need a replacement by the 70's at the latest, assuming an early 50's commision date. The they will also become to man as the navy shrinks from the 60's onwards.
I thought it might be something along those lines. Unfortunately, I dont know much about aircraft carriers, but it would be interesting to hypothesize what effect the carrier and its class might have on Maltese independence later on. Probably no real effect, but knowing us it would have steered Malta to be a bit more pro-British rather than pro-American.
I rather suspect that if the Malta's are built (unlikely) it would come at the expense of the Audacious Class. Since the Audacious Class were all renmaed during construction to honour British carriers lost in action during the war, then the Maltas may also be renamed as such - HMS Eagle, HMS Ark Royal, ect.
Russell