I would check with your clinician or NHS clinic (especially if it's associated with a university or research facility) to find out if they're going to participate. I personally believe that health care providers would be more interested in ResearchKit to drive specific studies than in HealthKit to monitor the more superficial activity, etc., for what it's worth...
You could also create a US-based app store login with no credit card for free apps - that's how I was able to get OpenTable UK from the US... Google for the instructions - you have to do things in the right order to avoid being asked for a US-based credit card... That may get you the app, but not necessarily full access to whatever the app does.