You could just double the cost.
The most interesting idea I heard about transports, was to create a second variety of transports which had movement factor 0. These would be the cheaper variety of transports which could only be used for rebasing missions: in other words UK moving its units from UK to Durban, and then Durban to India, for example. Then there would be the more expensive landing craft that would be capable of short runs, and landing troops on beechheads.
The problem would be that the 0 movement factor TP's would could get caught out in mid-atlantic, for example, and simply have no way to retreat. Distaster would ensue, if the TP's met a sizable force, but that is not so unrealistic, IMO.
Making an artificial limit on TP's would really kill the UK, since there would be no way it could effectively move units from theater to theater.