It likely is, I wouldn't think that they would keep separate frames for both brands.
The GP had plastic Simplex gear changers though and heavier or wider rims?
I've got a Grand Prix in the parts pile and its rims are marked Dunlop not Sturmey Archer and its got a GB stem and bars and a 14/28 Atom Freewheel.

I suppose the use of the narrower S/A rims, AVA hollow stem, Huret derailleurs, Brooks leather saddle, and the 14-24t freewheel were a step up from the Raleigh GP?
Side by side the forks are very different, the fork blades on the GP are longer and more curved at the tip, the Carlton's forks are straighter, have far more tire clearance, and the crown appears to be cast not stamped steel as on the GP. The forks on the Corsa Strada were clearly made so they could accept a wide range of fenders and tires.
The GP also has Weinmann Brakes with turkey levers where as the GB levers don't appear to have that option or ability. Both are 23.5" frames and the Carlton sits and rides much taller, likely due to the fork shape and length. The chainstays are slightly shorter on the Carlton as well. Both have the wrap over seat stay design frame, but the thin part that wraps over the top is thinner on the Carlton frame by about 1/3.
I don't suppose this Corsa Strada has had much actual use over the years. considering the tires are still Dunlop, and all the cables appear original, as does the bar tape and saddle, its likely it got hung up in the barn where it was found a long time ago and never touched again.
Its got little to no rust, plenty of dust, and so far, no pits or permanent marks on any of the chrome. I highly doubt anyone did any parts changing once it left the dealer back in the day.