There is a cutaway drawing by Theo Page from Autosport which shows, as Ray points out, the spare wheel/tyre mounted above the ‘passenger’ leg space and partly over the driver’s left leg.
To require just that very small blister to clear the upper edge of tyre, I was expecting to see that spare wheel sloping down leftwards into the left hand footwell at an angle, but it appears to be near horizontal in the drawing. The bodywork there is slightly concave so it might have just cleared on the left, but If the tyre didn’t need a clearance bulge at the forward edge too, I assume it must actually slope down a touch towards the front. But the clearances must be extremely tight! Difficult to see what else that bister might be for though...
(Other, but not all, pictures of BT8s I can see online exhibit this feature.)