There's a FAQ Page on the sidebar that will answer most of your questions.

In short, you need to have tried 2 or more traditional treatments to be able to qualiy. If you're applying for depression that's antidepressants or talking therapies referred by your doctor. If you're applying for pain that's 2 or more painkillers prescribed by the doctor or physiotherapy.

If in doubt, you can apply to clinics like Zerenia who will give you a free appointment with a patient coordinator to discuss your eligibility and answer any questions you have.

Remember that it's all private so there are clinic fees, repeat prescription fees, and the cost of the medication itself. You will also have the cost of a vaporiser device, since smoking it isn't legal, nor is it cost effective.

Using Zerenia as the example, It's £80 to register, £40 for a follow up appointment every quarter (Includes your prescription for that month, and otherwise £15 for a monthly repeat prescription. The cannabis itself starts from around £5 per gram on top of this, and is available packaged in containers of 5 or 10 gram denominations.