Welcome to the forum.
Personally, I wouldn't wait but it shouldn't take 3-4 weeks, mine took 4 days to complete import and registration formalities from port. I'd give the dealer a firm nudge on that.
According to my dealer, all stock they've received from late October has been fitted with rear LEDs so it would be unlikely that yours would come fitted with the previous style light cluster.
Regarding getting a new reg, I've never really understood the enthusiasm for getting the latest plate (I'm not from the UK). Having a new plate draws attention from vindictive and jealous sorts, the type who run keys down the side of cars but yes, I get the valuation argument but just let the car do the talking!
