Congratulations Bikerbluz, that's a cool bike! Good condition all round.
These were the last Reynolds 531 frames built at the Nottingham plant. After the last Lenton Grand Prix left the building, Raleigh didn't build with 531. When they started putting the Raleigh name on frames again, summer fine years later, they were made at the Carlton plant in Worksop.
I had one a Lenton Grand Prix as well, probably a 1958 one. It was pretty tatty but I loved it.
The early ones (1958-?) had a lever operated Cyclo Benelux front derailleur, so only one brazed on shifter boss. I really liked the lever operated derailleur, in some ways one of the best front derailleurs I've ever used-- though of course it was shifting between 46t and 49t chain rings, and i really couldn't tell the difference.
The original rear derailleur, a Cyclo Benelux four speed model, was trouble. They shift well if adjusted right, but no matter how stoutly built (a lot of steel!) they would bend if the bike fell on the right side. Once bent, the derailleur would shift into the spokes automatically. That's likely why you have a Frame rim and a Svelto derailleur!
The original rims were Dunlop Special Lightweight, 27". They were chromed steel but no heavier than most aluminum ones and really the nicest steel rims ever. No matter how badly they got banged up, you could true then again.
Looks like you have the original Racelite hubs, which are excellent.
The earlier ones had a Williams chainset (crankset) with Raleigh herons on the outer ring. I'm not knocking your crank, though. It is a Raleigh product, also quite nice. The Gran Sport you mentioned had the same crank.
I am puzzled by your brakes. I think those are the (very desirable) 1950-53 model. I believe the original brakes on your bike were more likely Weinmann 730's. Needless to say, I could be all wrong about that.