From what I read around, you can get Sky installed without a phone line with something like a Â£25 surcharge for the standard Sky box with a new subscription with installation. Sky+ and Sky HD both require a permanent phone line connection going by their contract, but from reading around, it is not strictly enforced and read about people even activating their boxes successfully without them ever being connected to a phone line.
Going by experience from people I know who have Sky, some have received letters from Sky asking them to plug in their Sky box once left unplugged for a month or two, but no one has had any other issues or threats from Sky. However, the only ones I heard about getting legal threats from Sky were those with multi-room viewing failing to leave them connected to the phone line, but this is understandable since Sky closely guards these to ensure all boxes part of the multi-room subscription remain in the same building.
It would be worth getting someone in the UK to ask Sky if they can get the dish and Digibox without installation, as this way it would save having to get the equipment installed only to be removed later. Alternatively, you can get a Sky Digibox 2nd hand or from eBay and have someone in the UK order a Sky subscription for it. All they will neeed is the box’s serial number and box type (Sky, Sky+, HD, etc.) to provide to Sky.