The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 3, 'Names'
In a historical context, relevant anglicized or obsolete names may be appropriate (e.g. Bombay, Danzig, Rhodesia), but otherwise current usage should be respected (e.g. Mumbai, Gdańsk, Zimbabwe).
Where there is a current English form for foreign names of towns or cities (e.g. Cologne, Dunkirk, Florence, Geneva, Lisbon, Majorca, Mexico City, Moscow, Munich, Naples, Quebec, Salonika, Turin, Venice, Vienna), it should be used. Obsolete English forms (Francfort, Leipsic, Leghorn, etc.) should be avoided. The forms Luxembourg, Lyon, Marseille, Reims, and Strasbourg are now more widely used than Luxemburg, Lyons, Marseilles, Rheims, and Strasburg or Strassburg and are therefore recommended.
The following are now the official spellings of certain Welsh names (including in texts written in English) and should be used instead of the anglicized forms found in earlier maps and in earlier books: Aberdyfi, Aberystwyth, Betws-y-Coed, Caernarfon, Conwy (river and town), Dolgellau, Ffestiniog, Llanelli, Tywyn.
The use or non-use of hyphens in names such as Newcastle upon Tyne, Stratford-upon-Avon should be checked in a good reference work. French place-names are regularly hyphenated, e.g. Colombey-les-Deux-Églises, Châlons-sur-Marne, Saint-Malo, except for an introductory definite article, e.g. Le Havre, Les Baux-de-Provence.
Note the correct form of the name of Washington, DC (comma, no stops).
Distinguish between (a) Great Britain (i.e. England, Scotland, Wales), (b) the United Kingdom (England, Scotland, Wales, Northern Ireland), (c) the British Isles (England, Scotland, Wales, Ireland, the Isle of Man, the Channel Islands).
(a) that England should never be used for any of the above;
(b) that the term Britain in its strict sense is the equivalent of Great Britain but is so extensively used as the equivalent of the United Kingdom that it would be pedantic to object to its use in that sense;
(c) that the Irish form Éire should not be used in English as the name of the Republic of Ireland;
(d) that the Isle of Man and the Channel Islands are not parts of England, of Great Britain, or of the United Kingdom.
The definite article is no longer used in the names of the countries Lebanon, Sudan, and Ukraine (but the Gambia, the Netherlands).