I reckon the age thing would be less of an issue if you bought it from a small, local general store in the more hilly areas of UK. I know a lot of them do sell meths. There's one on the way to Wasdale in the Lakes for example that sells all the usual provisions plus outdoor gear, gas, stoves, meths and other stuff you might need such as ear plugs. As well as quite possibly being less stringent on the age requirement for meths camping fuel you would be supporting a local business which is very important in these more rural areas IMHO. IF they do prevent you buying it then just wait outside with your ruckack on and ask their next customer to buy it for you.