You would need a prescription to buy insulin in the uk. I suppose one possibility would be to see a private GP for a prescription, don't know how much that would cost, or whether they would dual prescribe.
What insulin are you using? In what kind of doses through the day? Are you on any other medication? What's your daily diet like? And what's your BMI?
Are you having hypos? Did the "too much insulin" determination come just from a prescribing clerk, or actually from your GP?