You absolutely do not need to lower your car.
You might want to for aesthetic reasons, but it is not necessary for fitment of the larger wheels.
If you're doing it right and 'plus sizing' the tyres (fitting lower profile to the larger wheels to keep the rolling radius the same) then you will take some of the suppleness out of the chassis as the tyres will have stiffer sidewalls. By lowering the car you will lose further compliance in the ride - it'll feel like a demon on a billiard table surface, but you'll have to slow right up and drive like a girl if the road gets rough (which it does a lot in the UK).
At the end of the day it's entirely up to you. If you like driving then lowering might just put a dampener on your fun.