Hi Sarah,
Yes there is an "excess" or "deductible" on basic cover. You can choose your deductible and people in general good health go for a high one as the premiums are so high with the low ones that it actually works out cheaper to pay for the odd doctor visit yourself and save on monthly premiums.
For glasses, basic cover (the compulsory/expensive bit) will cover you 180.- every 5 years. Yoopi...
With supplementary (the extra, non-compulsory bit) every company is different. I'd say an average would be 200.- every 3 years although some companies will cover you 250.- every year, some cover 100.- per annum which you can cumulate up to a max. of 500.- after 5 years.
I usually advise people with contact lenses to go for the 250.- p.a. option as, although it's slightly more expensive per month, you're getting more back in the long run.
It's rare for those of us who only wear glasses to change every year so I tend to recommend either a less expensive option or a different company. Depends on the other needs of the person.
For more info on deductibles/excesses/franchises :
"What is a 'franchise'?
This is an amount of money that you elect to pay before claiming on your health policy, similar to 'excess' or 'deductible' on car insurance in order to reduce the cost of the premium.
It is worth bearing in mind that the franchise operates on an annual basis and not on a per-case basis. Therefore should you have a claim that crosses two financial years then you will pay two separate amounts even if it is for the same illness or accident."
Heather explains the system pretty well at the beginning of this post