Sorry that I can't help you with where to take him in the UK, but honestly an x-ray can tell them so much more about a leg issue than blood tests. Sounds like he really could use it to see exactly what the problem is since nothing else has helped. Hopefully someone from the UK will see this and have some suggestions for where you could maybe take him for cheaper. Are there any rescues in your area you could contact for help?
I can tell you really care for your duck, so I truly hope you can find some help for the poor guy. I have a duck with a special leg as well (ever since she hatched). I tried everything to fix it as best I could when she was little, but she still has problems with it. She is on long-term pain meds to help with any discomfort she might have, but she can still get around just fine. She just has a permanent limp and is a bit more clumsy than the rest. She is the sweetest duck I've ever had, and loves to snuggle in my lap every day (her baby picture is my avatar).
Good luck with your special duck!