The British Isles are a cluster of islands located off the north-west coast of continental Europe. The largest islands in the group are Great Britain (so called to distinguish it from Brittany in France, which covers a smaller geographical area, rather than because it's particularly great in itself, although of course, it is) and Ireland. Great Britain is divided into three countries: England, Scotland and Wales, all of which are currently constituent parts of the United Kingdom. Ireland is divided into Northern Ireland, which is also a part of the United Kingdom, and the Republic of Ireland, which isn't.
For the sake of Americans and other strange alien visitors, what all this means is that anyone born in Great Britain or Northern Ireland is British, but depending on their specific location, they may also be English, Scottish, Welsh or Irish. Despite the apparently common misconception of many foreigners, however, the terms 'British' and 'English' are not interchangeable, and using them in such a manner tends to irritate people from Scotland, Wales and Northern Ireland. Just to add to the confusion, people from the Republic of Ireland are not British, despite the fact that Ireland is clearly in the British Isles. Inhabitants of the Falkland islands in the south Atlantic, however, are British, as are the population of Gibraltar, despite the fact that neither of these places is anywhere near Britain itself. Clear? Good.
In terms of the comics world, Britain is a fairly diverse place, with a number of locations not generally to be found in the so-called real world. In the fictional universe(s) created by Dundee-based publishers DC Thomson, probably the most significant locations are Beanotown (home to the characters populating the Beano) and neighbouring Dandytown (home to most of the stars of the Dandy, excepting Desperate Dan who lives in the bizarrely British-influenced American town of Cactusville). The precise locations of Beanotown and Dandytown are unclear, but they can probably be safely assumed to be somewhere in the in north of England, as that is the location of most of the numerous grimy industrial towns which are home to the various characters in DC Thomson's now mostly defunct adventure titles such as Wizard, Rover and Hotspur. Some vague part of the north also seems to be the location of Croynge, the setting of the Daily Mirror's long-running cartoon strip The Perishers, although the name of the town is actually an amalgamation of Croydon and Penge, both locations on the outskirts of London.
London itself, of course, is the home to a number of comics stars, though nowhere near as high a percentage as the quite staggering number of American characters who seem to live in New York. Marvel UK's flagship character, Captain Britain, was originally based in London, studying at Thames University, though he soon relocated to his ancestral home of Braddock Manor in Malden. (The good Captain, along with numerous other Marvel UK stars, also has strong links with Darkmoor, a remote region which appears to be somewhere in the north of England, near the Scottish border). Tharg, the alien editor of 2000 AD, also had his original headquarters in London, specifically Kings Reach Tower, as did the denizens of the Marvel UK Bullpen, in Jadwin House, which cropped up frequently in Bullpen Bedlam, one of the humour strips based on Earth 33 1/3. Thamesford F.C., the original home of Striker star Nick Jarvis, is also in London, though the precise location of Nick's later home, Warbury, is anybody's guess (but probably somewhere in Yorkshire if club owner Eric Openshaw is anything to go by). In one particular comics universe, London was devastated by the insane Kid Miracleman. The mutant team Excalibur were based in London for a time, but later moved to Muir Isle, off the coast of Scotland. Another rather unusual émigré to Scotland was the improbably-named time traveller Kilgore Slaughter, in the strip Timesmasher.
Cornwall, in the south-west,, was briefly home to the amnesiac Captain Britain (Brian Braddock), as well as the witch Old Sarah Mumford, and neighbouring Devon is the location of Great Tor, home of the Grappler from Great Tor. Meanwhile, on the far side of the country, Ipswich in Suffolk was the home of Brian Braddock's temporary replacement as Captain Britain, Kelsey Leigh. Eagle Street in Ipswich, home of the venerable Central City Comics, was once the site of a supervillain rampage halted by the Avengers, echoing an earlier incident in the 1980s when the original Forbidden Planet comic shop in London's Denmark street was almost trashed during a fight between Captain Britain and Slaymaster! That never happened to Dark They Were, and Golden Eyed...
Another notable location is the village of Stockbridge, somewhere in the south of England, which for a time became a kind of second home to wandering time traveller the Doctor, and was the actual home of his associates Maxwell Edison and Izzy Sinclair. Birmingham, in the west Midlands, was also briefly home to an alien visitor in Alan Moore and Jim Baikie's minor masterpiece Skizz.
In the distant past, at least according to the mythology of Sláine, the British Isles were all a part of a much larger area called Tír na nÓg, which covered much of Northern Europe, while in the future, in the worlds of Judge Dredd and Robo-Hunter, much of Britain becomes one of a couple of versions of a huge Mega-City called Brit-Cit (Ireland, in Dredd's world, has now been renamed the Emerald Isle, while Scotland is Cal-Hab). These are, of course, only two of many potential future Britains found in comics, with others being found in stories such as Mean Arena, Ro-Busters, Third World War, Invasion! (in which Britain is conquered by the Volgans, who are very definitely not thinly disguised Russians), Disaster 1990! (in which the country is flooded when the polar ice cap melts) and Strontium Dog (which introduced us to the mutant ghetto of Milton Keynes!).
Meanwhile, a neighbouring dimension to which the British Isles are magically linked is Otherworld, the location of Britain's mythological landscape, and the current location of Avalon, Camelot and the headquarters of the Captain Britain Corps.
The UK is protected by the paramilitary group U.N.I.T. and by various secret organisations such as S.T.R.I.K.E. (Special Tactical Reserve for International Key Emergencies) and the R.C.X. (Resources Control Executive), as well as by superheroes such as The Vigilant, Union Jack, King Cobra, Zenith and the Knights of Pendragon.
Basically, there's a lot going on here!