It's very hard to get a shotgun certificate in the UK - well, in England/Wales/Scotland, it's a little easy (but not easy) to get your hands on a gun in Northern Ireland obviously, the laws are slightly looser there (eg handguns are allowed and self-defence is allowed as a reason for getting a gun).
You must disclose any relevant physical or mental health conditions that you have been diagnosed with or treated for in the past as this may affect your ability to safely possess and use a firearm or shotgun.
Re: can i get a licence with ADHD? It doesn't exclude you, but ADHD is classed as a mental illness, so you will need to declare it in the application. They may ask you to get your doctor to complete a medical assessment before making the decision.
Fees for certificates in England, Wales and Scotland Licence typeFee Grant of firearms certificate £198 Renewal of firearm certificate £131 Grant of shotgun certificate £194 Renewal of shotgun certificate £12610 more rows