Some tandems (some single bikes, too) fold down, or come apart, which makes it possible to transport them inside most ordinary sized cars. These sorts of tandems are astonishingly expensive, as you might imagine. Try Bike Friday.
Otherwise you're looking at getting the tandem onto the roof of your car. Some people achieve this using ordinary roof bars and a lot of rope (lie the tandem flat). Else you'll need a roof rack. There are only a few models available to buy in the UK. Try Roofrack for Pendle models, or Google for the Atera Tandem Bike Lift. The Atera costs a lot more, but makes getting a tandem up relatively easy.
Tandem racks occasionally pop up on Ebay, but price may not be much less than new. Otherwise, try joining the Tandem Club and posting a wanted ad if you're after a 2nd hand roof rack.