GPs can't prescribe ADHD medications (stimulants). You need to ask for a CAMHS referral. If you are too old, ask if they have an adult ADHD diagnostic service (unfortunately provision in the UK is poor).
If you have a couple of hundred pounds spare, you could pay for an assessment and potential prescription yourself.
If your difficulties are well documented in school reports and doctors notes (or reports from any other agency) , but can't afford an assessment, you could attempt to claim PIP to fund it.