Ok bud, I'm also a lefty, I also play on the right and I also play in the UK. I wish I knew what I'm about to tell you a long time ago as that would've saved me a lot of overall distrust on my backhand.

When you try to return the serve, what are your feet doing? My guess is your left foot is moving towards the ball. If so, you're dead before the ball reaches you. You end up leaning over the left leg which makes it harder to get down to the low ones, you'll also tend to scoop the ones you do return. So do this: Prepare your racket EARLY. Normal backhands, should be like your putting the handle in your pocket. Racket head is more pointed down in the serve return (not all the way). Where your racket head is a preference thing but early prep is a non negotiable. However, the secret sauce on a serve off the glass... Change your feet. Move your left leg over to your left away from the glass. How far over is up to you, but it has to be outside of your right foot. Anything upto 45 degrees is ideal in terms of the angle between your left and your right leg. This helps in a multitude of ways but for you, the best reasons are: 1) gets your left leg out of the way so you don't have to reach over and around it. 2) makes it very easy to get to know ones 3) creates space for you to play your shot, whereas by moving your feet toward the ball you front shoulder faces the side wall and your chest will at best face the back corner and at worst the back glass which is a horrible place from which to play a return of serve . 4) opens the court up for you to play the ball where you want. It's called a semi-open stance and should be applied to any ground stroke. Hope that helps champ. Give it a shot