While Cornwall's Land's End is only mainland England's most westerly point (see Ardnamurchan Point for Great Britain's), its other claim to extreme fame is good for both. Lizard Point, at the southern tip of The Lizard peninsula, really is mainland Britain's most southerly point. For the UK as a whole you need to go to St Agnes, Isles of Scilly (as I did) and for the British Isles, it's hard to beat Les Minquiers, south of Jersey in the English Channel.
Other UK and British extremes include:
North Mainland: Dunnet Head, along the coast from John O'Groats.
North UK/British Isles: Muckle Flugga off Unst in Shetland.
East (all): Ness Point, Lowestoft, Suffolk.
West UK/British Isles: St Kilda.
And Rockall, way out in the North Atlantic!
There's a Bookmark list for more around the UK.
This extreme Letterbox cache (please do not remove the stamp from the cache) was originally set during GC3BCB7 - SP's Final Extreme? (Lizard Point, Cornwall) in June 2012.
Read more about The Historic Geocaches' campaign.