Good points made here and thanks for the info on the Italian forces.My point was exactly that you cannot say the available British troops would be enough if you don't know how many troops the Italians had, now can you?
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Now on to the Italian garrison. I have only incomplete information, but I can tell you that the Regina division was strongly reinforced, having to garrison many islands. In June 1940, it could count on some 25,000 men, counting the attached coastal defense units and army batteries. The figure does not include Regia Marina and CCNN personnel
The British can certainly outnumber each single garrison in detail, possibly even the Rhodes garrison itself – provided they can land a lot of those troops at once. If they can't, they can still overcome the defenses of some minor island, but I doubt they would be able to do so on Rhodes.
I happen to have details on the armored units. A poster said that the CCCXII Battaglione Carri had the Fiat 3000s. It was a Misto (mixed) battalion, and it actually had 4 M11/39s, 24 L3s (two 8-vehicle companies and 2 independent platoons; I suppose these would have been detached on other islands than Rhodes) and a recon platoon with 6 ancient 1ZM armored cars. The Fiat 3000s, some of which certainly in the B (cannon-armed) version, were 2 platoons belonging to the 3° Compagnia GAF (border guard), but most of these would have been broken down.
None of these are a problem for a Matilda Mk II – but then again, the question remains whether the British would be able to land these.
The British deployed almost 60,000 men to Greece in OTL, so if these forces could be made available sooner, they would stand a very good chance at taking most of the Dodecanese. I agree that taking Rhodes would be difficult with a direct assault, but I believe that the correct strategy for taking the Dodecanese would be to make a longer rather than a shorter campaign.
In other words, the RN would have to project power and cut the islands off. This should be doable and with the deployment of a couple of RAF squadrons to Eastern Crete the Italian Navy should be reluctant to project power there. If they did bring their heavy units out (those that were left following Taranto) they would probably see them sunk anyway.
Then in the beginning of 1941 small British forces would try to capture the smaller islands of the Dodecanese, while the Royal Navy would have to make sure that the Rhodes troops did not try to reinforce the other islands. With Kos, Leros or Karpathos captured, the British would have a new airport to operate out from in the local area.
These operations should take place in February-March 1941, combined with a builup of forces in the Dodecanese. Then in late April 1941 the British can try to capture Rhodes, which by then cut off for 3 months should be low of fuel+ammunition.
The Germans would surely arrive then, taking Greece by storm, perhaps a couple of weeks earlier than in OTL (bear in mind that a much sooner German campaign would have lots of butterflies on Bulgaria+Yugoslavia).
Once the Dodecanese were secured, the British can send some reinforcements to Crete.