The legislation:
http://www.legislation.gov.uk/uksi/2003/2243/made
The official guidance notes:
http://www.food.gov.uk/sites/default/files/multimedia/pdfs/honeyguidance.pdf
English, but Scottish, Welsh and NI have analogous legislation.
If it's called "Honey", then it has to be the natural product, that is made by bees collecting nectar. Most of the forage in the UK provides the sugars glucose (particularly OSR) and fructose and small amounts of sucrose. White table sugar is all sucrose. If you feed bees sugar in winter bees move it around the hive and if as a lot is left in spring some will end up in the honey they store the next summer. Hence the maximum permitted levels, some sucrose is natural and a little may be left over from winter feeding but adding sugar syrup that you know will be in what you later sell as "honey" is an offence.
There's a legal way to avoid the problem. That is not to call it "Honey", it used to be quite common to see "Honey Spread" or similar on the label. That could be blended with sugar and is still legal in some countries. As far as I recall I think that's now considered too close in the UK although you could probably use "Spread blended with Honey". Honey as an ingredient for "Honey Cake" or whatever where more sucrose is added is unlikely to be a problem.
The illegal way is to use one of the ways of reducing the sucrose content of your feed. A partially inverted syrup like Ambrosia is lower in sucrose. Some of the earlier marketing suggested inverted syrup was hard to tell from honey, the purpose of the phrase was rather ambiguous and is not in the current leaflets. In the US High Fructose Corn syrup is widely used as a bee feed, and has been used fraudulently to "extend" honey. There have been efforts to detect some forms of fraud in the UK
http://www.bbc.co.uk/news/uk-england-norfolk-24475094 although most of the recent publicity seems to be about faking manuka.
The official answer is that if you see "Honey" on the label and "blended from EU and non-EU sources" as the origin then it's the cheapest honey they could find on the world market. Could be from China or wherever labour is cheap and regulations lax, probably from sources where it was harvested early and artificially dried. Unofficially, if it's clear as water and has been heated and highly filtered (to remove pollen) then you have no realistic way of telling what it is or where it's from so it might as well be mostly corn syrup.
