It's only about 60% of people who qualify for free prescriptions (still a lot). But 90% of prescriptions go to those who qualify for free prescriptions (mainly because older people tend to need a lot more medical care and all those over 60 qualify).
At the moment people who qualify for free prescriptions should already have some sort of government issued paperwork that supports their eligibility. Means testing the whole population would be pretty expensive and so add to administrative costs of the NHS (not what were after!). We could use the fuel allowance means testing to determine eligibility for those over 60 which, I believe, would cut out those 60-65 and about 25% of people over 60 - probably not as huge a number as needed to make much difference, especially since we'd need some sort of frequent flyer certificate like we have with prescriptions for those with chronic conditions if we didn't want a bunch of people keeling over because they couldn't afford care.
The trouble with all these approaches is that research suggests the unintended consequences of charging for medical access at the point of access leads to missed and later diagnosis, mismanaged chronic conditions and more severe presentations which cost the health service more to treat and manage. This is why we have free prescriptions for such a large group of people. If those who currently have to pay for prescriptions also had to pay to access GPs or other care, we would likely see a big spike and it's workers who would be hit - the people who produce the wealth that pays for everything - lowering productivity and damaging the economy.
A more difficult to implement, but likely more effective plan would be to increase tax on everyone, even those on benefits, and have people able to get that tax discounted if they follow healthier lifestyles, get their vaccinations and screening tests, take a first aid course, etc. so that the burden on the health service is lowered and those who don't follow those guidelines pay more without it impacting their ability to access medical care when they need it. Would be a massive row over what would constitute a healthier lifestyle, though, especially since the UK is such a diverse country.