Make sure that the venison fits into your grill pan; if necessary, cut the loin in half to fit.
Mix together the peppercorns, juniper berries and some salt in a large, shallow dish. Dip the venison in the egg white, then roll it in the peppercorn mixture, covering it evenly all over.
Cook the venison under a preheated hot grill for 4 minutes on each of the four sides, turning it carefully so that the crust stays intact. Transfer the loin to a lightly greased roasting tin and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for another 15 minutes for rare and up to 30 minutes for well done (the time will depend on the thickness of the loin of venison).
Leave the venison to rest for a few minutes, then slice it thickly and serve with green beans, redcurrant jelly and finely sliced sweet potato crisps.