It is not illegal to eat horsemeat in the UK but to most British people it is a taboo food.
However, thousands of horses are slaughtered annually for human consumption overseas at three abattoirs in England:
Potters Abattoir near Bristol;
Cheshire Equine Services, Nantwich; and
R E Williams & Son, Weobley, Herefordshire.
FAOSTAT figures reveal the first export of 1606 tonnes of UK horsemeat to Europe was in 1963.
After that it steadily increased most years to a peak in 1982 of 7,781 tonnes representing the slaughter of 26,000 horses.
By 1997, the UK trade figures for "Meat of horses, asses, mules, hinnies: fresh, chilled, frozen" showed that 2,515 tonnes were exported, mostly to France. In 2000 it was 2857 tonnes (9,500 horses) mainly to France but some to Belgium. After that the quantity declined to 1,576 tonnes exported in 2004. 
Currently DEFRA estimates that between 6,000 and 10,000 horses are being killed annually for consumption abroad.
A spokesperson for Potters Abattoir estimated that 80% of the horses they receive are Thoroughbreds or thoroughbred types, 10% of which come directly from the racing and breeding sector, some after a varying period of retirement.
A further percentage comes from the 4000 or so Thoroughbreds bred in the UK every year with insufficient ability to be worth racing or breeding from,  which take a circuitous route to slaughter via a sports and eventing career first. Still others would be the retired but still sound racehorses of suitable type and temperament previously sold on to other disciplines and which in time also take the slaughter road.
The abattoirs pay as much as ₤650 for a large Thoroughbred,  an attractive proposition to owners faced with the major expense of disposal by other means, anything from £150 for collection and processing by a renderer to £750 or more for home euthanasia and cremation.
Although it is not actually illegal, there is no live export of horses from the UK specifically for slaughter.  The reason is that a mandatory minimum value above that which horse slaughterers would pay is placed on any horse exported abroad, though UK horses of higher value are regularly exported for competition and breeding purposes and no doubt end up joining the slaughter trail in Europe.
Additionally, it became compulsory in 2005 for all equidae in the UK to have a passport detailing any medications to ensure that certain drugs do not enter human nutrition through horsemeat. No horse, pony, donkey or mule can be moved out of the UK without one.
Since March 2006, horses sent to abattoirs in England had to have a passport before they could be slaughtered. This caused a substantial drop in business, though probably not in number of horse deaths, the surplus being disposed of in other ways. 
Faced with the continuing problem of what to do with the unwanted horse, major organisations like the ILPH and RSPCA agree that the horse slaughter industry provides an effective and humane disposal service, provided there is stringent adherence to welfare guidelines. In the words of one abattoir owner:
"The level of welfare abuse of horses in the UK is but a shadow of that which would occur should an abattoir trade not exist." 
 Agricultural Data on FAOSTAT | http://faostat.fao.org/
 uktradeinfo website, Comcode 02050020 | http://www.uktradeinfo.com/
 Personal communication Customer Contact Unit, DEFRA, UK
 Personal communication Stephen Potter, Lawrence J Potter (SW) Ltd, Bristol
 "What happens to unwanted horses?" The First Post, 31 July 2006 | HTML | PDF
June 18, 2005
Link to Report »
"The slaughtered horses that shame our racing"; The Observer, by Antony Barnett; Oct. 1, 2006.
QUOTES FROM UK SLAUGHTERHOUSE WORKERS
"We use a captive bolt if we can get close, if they are quiet, but we use a live bullet if the horse is a bit “nasty” because you can stand further away".
Substitute 'panic stricken' for 'nasty'. Then use your imagination.
Asked about horse slaughter a rather taciturn man said [with] pity in his eye, “yes terrible” he said, "you could see how frightened they were, neighing, ears back, eyes rolling, they were scared you see, they could smell the blood and sense the fear!"
QUOTE FROM DR. ROBIN COOK MD
“. . .the most terrible thing I have ever seen, both as a medical doctor and as a man”.
“The slaughterhouse workers seeing this once powerful and majestic animal in it’s last gallop of death callously cheer it on.
“The noise is deafening – the smell even worse, the horse has been urinating all over [him]self since being led unwillingly into this horror, the smell of acrid urine manure and blood – all mixed together makes the senses reel”.