A pilot may only fly microlights as Pilot In Command in the UK if they hold valid medical certification.
Until recently, the The NPPL medical certification needed to be countersigned by the pilot’s General Practitioner (GP), and required renewing on a regular basis.
Now the rules have changed, and as of 7th November 2016, a self certified form is all that is required. Basically, it is the same rules that apply to driving, so if you are fit to drive to the airfield, you are fit to fly! Forms can be emailed to the CAA, and a receipt will be sent back to you to confirm delivery. All pilots are strongly encouraged to submit the forms as soon as possible, regardless of whether their existing certification is still valid for a while.
Link to the medical form: LINK