Hey, congratulations that you can go straight on a bike, even if you are wobbly. Practice is the secret to success! You can do this! What I suggest is: bring your bike to a large empty car park. Wear long sleeve shirts, long pants, and a helmet. Mount the bike and go straight. Now to do a turn, you do not really move the handlebars. Instead you lean your weight in the direction you want to go -- gently of course -- and the bike will go where you point it, sort of. This will let you do a big gentle circle and help you build confidence. You should be able to practice riding in large circles for an hour. Do an hour every day for a week. After a week, try putting up some cones or whatever you have available to mark a left turn. See if you can do that left turn without your feet touching the pavement. Don't go fast at all, just concentrate on doing the turn. If you fall, that is okay, we all have falls now and then when learning how to ride. Pick yourself up and try again. Practice left turns for a week. At the start of the third week, practice right turns. Keep practicing turns like this for a week. Then make it more complex: do turns from a stop, both right and left. Practice doing roundabouts. Remember to expect to fall down every now and then, it just happens.
It may also help you to have a friend with you who is an experienced cyclist. I'm sure your friend can improve on my suggestions.
Note for forum members who have never been to the UK: remember, they drive on the left over there. The British system of roundabouts is a lot more efficient than our turn signal systems. It's easier for "Sheriff" and other riders in the UK to make a left turn than a right turn because a right turn often signifies crossing a lane of opposing traffic.
I hope this American-who-has-been-to-the-UK is remembering it all right.
Bob