Preheat the oven to 200C/180C Fan/Gas 6.
Loosen the skin on the turkey crown by pushing your fingers (or rubber spatula) between the skin and the meat, moving it around to get to the tricky places and taking care not to tear the skin.
Mix the thyme leaves with the softened butter until well combined, then smear the mixture underneath the skin of the bird. Arrange the orange slices in two neat rows under the skin, on top of the herb butter.
Place one of the orange halves under the skin at the neck end of the bird, and any orange trimmings in the neck cavity.
Transfer the turkey crown to a small roasting tray. Rub all over with the oil and season with salt and freshly ground black pepper. Roast in the oven for about 1½-2 hours, or until the juices run clear when the turkey is pierced in the thickest part with a skewer and no traces of pink remain. During cooking, check the turkey every 30 minutes, baste occasionally and cover with aluminium foil if it is browning too quickly.
About 15 minutes before the end of cooking, squeeze the juice from the remaining half-orange over the turkey. Return the turkey to the oven, uncovered, to allow the skin to crisp up.
Transfer the turkey crown to a serving platter and set aside to rest, covered in foil, for 30 minutes. Reserve the meat juices left in the roasting tray.
For the gravy, pour the juices from the roasting tray into a jug and allow to settle. Skim all but 4 tablespoons of the fat from the top, then tip the remaining gravy into a saucepan and heat over a medium heat until the gravy is just simmering.
Whisk in the flour until the mixture is smooth and well combined, then pour in the port and stock and whisk again until smooth. Continue to simmer the gravy until it starts to thicken, then add a few drops of Worcestershire sauce and the soy sauce.
Bring the gravy to the boil, adding any remaining turkey juices released while the meat was resting. Season, to taste, with salt and freshly ground black pepper, then decant into a warmed serving jug and serve with the turkey.