Gazetteers & Placename Lists for the British Isles, including Ireland

English Place Name Survey volumes are a mine of detailed topographic information and are full of detailed discussions about etymologies for anyone studying the Arthurian period.

Ordnance Survey's Landranger gazetteer

Mid-19th-century OS maps gazetteer (Old-maps.co.uk)

Gazetteer of British Placenames (Assn of British Counties)

Archi archaeological sites index (Popular Archaeology) &, UK placename finder:

English Heritage Properties & Sites

CARN etc. (Royal Commission for Ancient and Historical Monuments of Wales/ National Monuments Record of Wales; registration required)

Gazetteer for Scotland (U Edinburgh, Royal Geographical Society of Scotland)

Gazetteer of Wales (GeogData)

Gazettees of England/Wales and Scotland--browsable lists (Coal Authority)

England and Wales places, 1891 census

Gazetteer of Markets and Fairs in England and Wales to 1516 (Samantha Letters for Centre for Metropolitan History):

British Isles Parish Locator (GenWeb):

Parishes in Scotland

The Scottish Place-Name Society was only founded in 1996 but will still be beneficial for use

IreAtlas Townland Database (Sean Ruad & Leitrim-Roscommon Genealogy)