Q: What's the correct distance for fruit trees?

I think it depends what kind of rootstock it's grafted on to, which decides how big it will grow. If it's gonna be a full size tree it needs a lotta space. but if it's half sized or dwarf it will need less. Hopefully someone will come along who actually knows all about it.

As Zelenina said, it will depend on the rootstock. I have recently ordered some apple trees too and I am using this page as a guide:

Fruit tree spacing and planting distances

It also depends on how you are going to let it grow. I have a load of cordons along a fence planted very close together but a full tree needs much more.

There isn't a definite answer.
There are lots of variables even across the UK, such as:

Your local climate - amount of rainfall, temperatures.
(milder and wetter climate makes a tree grow faster - although the wet can increase disease problems)

Your soil type, its depth and its fertility.
(good, deep, fertile soil makes a tree grow faster, and certain varieties cope better with more difficult soils)

The rootstock onto which your tree is grafted.
(some rootstocks make rose-bush size trees while others make trees as large as a house)

The variety grafted onto the rootstock.
(some varieties grow faster than others)

Whether the variety likes your conditions
(some varieties like hot, some like cool, some like wet, some like dry)

How often you intend to prune it.
(unpruned trees can eventually get very large)

The shape/form you train it into.
(trained forms are the size you want them to be)

Whether you give extra feed or water.
(more nutrients and especially more water make the tree grow faster)

Whether you spray for pests/diseases.
(it takes energy from the tree to fight pests/diseases, so the tree grows slower)

The natural disease resistance of the variety, and the types of disease in your area.
(a tree grows faster if it has more resistance to your local pests and diseases - which vary a bit by location).

If you want a tree to reach 15ft, you're most likely to need to be looking at one grafted onto at least MM106 or M116, and more likely MM111 or M25 - depending on all the factors I mentioned above.

If you've just bought a tree, the rootstock code will give a clue as to how big it might get.