MM, we've got friends in Dorset - around Blandford - and I give them seeds. They can grow beefsteaks in an unheated greenhouse. In a good summer, obviously, like last year. In ordinary summers they struggle to get a crop.
I've had a look around the UK seed suppliers for some nice varieties. I know nothing about this supplier's history or reputation but they have two excellent varieties - Anna Russian and Cherokee Purple. Both highly recommended.
Anna Russian is a dark pink heart-shaped indeterminate variety. My wife's favourite, I grow it every year. Hearts usually take a while to mature but AR is earlier than usual and is also much more prolific than a typical heart. Lovely rich but delicate flavour. The plant never looks sturdy; in fact, it looks like it could flop over at any time, but looks are deceiving. While it needs lots of tying up, it's as tough as old boots, its branches spreading like octopus tentacles.
They call Cherokee Purple a beefsteak but it's more of an oblate. Regardless, it's a delicious rich complex flavour. I gave a friend in Umbria a plant and he reckons it's the best tom he's ever tasted. I wouldn't go that far but it certainly gives your taste buds a real kick. Indeterminate, but never much more than a medium-sized plant.
They also list a couple of Costoluto varieties. I think they're the Italian beefsteaks Monty Don grows. They're the best-known Italian beefsteaks outside Italy. I've tried them a couple of times over the years and found them bland. There are many better varieties around.