Yes, I'd have thought a 307 would struggle with a J21. It would probably be just about OK on the straight and level, but any serious hills or slipways would be problematic. You are realistically looking at a towing weight of around 1500kg all up.
I think you need a medium 4x4 to be comfortable, with a 2 litre diesel minimum, something like a Freelander TD4, Kia Sportage at a guess. I use a 2.5 Sorento and it tows my
Dehler 22 like there is nothing there.
When I bought my Jag 21, I was told by the previous owner that he had taken it on a weigh bridge and it was 1750Kg. That was with the standard trailer. I towed my one with a Land Rover Discovery 2.5 TD5, towed like a dream, more stable than our 1300Kg caravan.