Here's what HM Revenue & Customs has to say about it, within Alcohol Duties guidance (added emphasis mine):
Wine is defined as a drink produced by fermentation of fresh grapes or grape must.
Made-wine is any other drink - apart from beer or cider - containing alcohol that is made by fermentation, rather than by distillation or any other process. For example, mead is classed as made-wine.
Fortified wine or wine mixed with spirits
If you produce any of the following, they are classed as spirits and you must pay duty at the Spirits Duty rate:
- wine or made-wine fortified to a strength above 22% ABV
- a drink that is classified as a spirit rather than a wine - for example, because most of the volume or alcohol of the final product comes from the spirit element rather than the wine
Generally, mead is not fortified, as alcohol has not been added to increase the content beyond that which is achieved through the fermentation processes. What you have is within the normal range for mead; it may make it strong, but it is not fortified.
Referencing the information in your own link, places your mead (12-14% alcohol) within allowances of 4 litres.
How much you can bring depends on the type of drink. You can bring in:
- beer - 16 litres
- wine (not sparkling) - 4 litres
You can also bring in either:
- spirits and other liquors over 22% alcohol - 1 litre
- fortified wine (eg port, sherry), sparkling wine and alcoholic drinks up to 22% alcohol - 2 litres
You can split this last allowance, eg you could bring 1 litre of fortified wine and half a litre of spirits (both half of your allowance).