Historically churches (Catholic, Anglican, Orthodox and so on) are dedicated to one of the saints or a particular mystery in Our Lord's Life or the Most holy Virgin Mary.
As for the Catholic Church, a bishop is free to give a church any title he wishes as long as some basic guidelines are followed.
Canon law requires that our sacred buildings (churches, oratories and private chapels) must be blessed or dedicated and given a title of (1) the name of the Trinity, or (2) the name of Christ, invoked under a mystery of his life or under his name already used in the Mass, or (3) the name of the Holy Spirit, or (4) the name of Mary, under a given title already found in the Mass, or (5) the name of the Angels, or (6) the name of a canonized Saint in the Roman Martyrology, or (7) the name of a Blessed provided the Apostolic See has given it’s permission. - Canon Law Guidelines for Naming a Catholic Parish
When a suffix is added to a title of a church, it may be either a reference to the saint or the place where the church is located or both.
Some examples of Church titles:
Church Domine Quo Vadis
St. Joseph the Worker Church
Hagia Sophia (Sancta Sophia or Sancta Sapientia)
Église Saint-Martin aux Champs (1220)
St James of Compostella (while its origins are not certain, Compostela may come from the Latin campus stellae, “field of stars”).
As for Catholic Churches that contain distinguished relics at an altar, I am sure that there will be special norms that are to be applied to the naming of the church or a particular side altar. (I know where to look, but have difficulty obtaining the required link, but will update when possible).
Now back to the question of as to Why is St Martin “in-the-Fields”?
In very early times it is said that a chapel dedicated to St. Martin was erected near Charing Cross, "for the convenience of the officers of Westminster Abbey and Palace, on their way to Covent Garden;" and this, no doubt, was the original "St. Martin's-in-the-Fields." But this is only a tradition. More trustworthy is the statement that St. Martin's was built by order and at the cost of Henry VIII., who disliked to see the funerals of his liege subjects passing through or past Whitehall, much as Louis XIV of France resolved to build the Château at Versailles because he could not help seeing the towers of St. Denis from the terrace at Saint-Germain.
The church is so called after the chivalrous Hungarian, St. Martin, who was Bishop of Tours in the fourth century, and in whose honour it is dedicated. It received its surname, "in the fields," like its sister church of St. Giles, from its situation outside the City proper, when it was first taken into the bills of mortality, in order to distinguish it from other churches eastwards under the same dedication. - St Martin-in-the-Fields
WEST VIEW OF THE OLD CHURCH OF ST. MARTIN'S-IN-THE-FIELDS; PULLED DOWN IN 1721. (From a Print published by J. T. Smith in 1808.)
When all is said and done a bishop may choose this title in honor of St Martin's field work of evangelization or after this saint to distinguish it from a previously named church of the same title (especially if more than one church is dedicated to St Martin in the area) or simply after anther church of the same name!. During the French Revolution thousands of French churches were destroyed. The French Church, Église Saint-Martin aux Champs was constructed in 1220 AD , and is clearly older than above mentioned British church!