In the United Kingdom, the date is written using the 'DMY' format, eg 21 October 2010 or if abbreviated, a stroke is used to separate numbers, eg 21/10/10 or 21/10/2010. The time can either be written using 12-hour, eg 4.10pm, or 24-hour notation, eg 16.10. Note, however, that 24-hour time is never spoken.
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English
Dates are written traditionally in "day month year" order, using a stroke as the separator. This order is used in both the traditional all-numeric date (eg "31/12/99") and the expanded form (eg "31 December 1999"). The Guardian Style Guide recommends you use this format (DMY; no commas). Writing the day of the month as an ordinal number (eg "31st December") is also very common (though writing eg "the 31st of December" is incorrect) - and since the advent of automatic correction in word processors, the ordinal indicator has been lifted into superscript (eg "31st December") in typed documents, to match the handwritten style. Single digit numbers for day or month may have a preceding nought (eg "09/09/2015"), but omitting it (eg "9/9/2015") is more usual in handwritten documents. You should not write, eg 09/09/15 because you are lengthening the day and month, whilst shortening the year.
When saying the date, it is usually pronounced using the ordinal number of the day first, then the word "of" then the month (eg "the 31st of December 1999"). The month-first form (eg "December the 3rd") was widespread until the mid twentieth-century, and remains the most common format for most newspapers across the United Kingdom. The month-first format is still spoken, perhaps more commonly when not including a year in the sentence, but is now less frequently used, having been superseded by the little-endian DMY format in most cases.
Welsh
The "day month year" order is also the case in modern Welsh (eg "20 Mai 1999", "20fed Mai 1999", "20fed Mai 1999"). The "month day year" order (eg "Mai 20, 1999") was previously more common than it is nowadays, it not being unusual to see a Welsh "month day year" date next to an English "day month year" date on a bilingual plaque from the latter half of the 20th century.
"20 Mai 1999" is read as "yr ugeinfed o Fai mil naw naw naw", with the usual soft mutation of M to F after "o" (of). 1999 can be read as either "mil naw naw naw" (thousand nine nine nine) or "un naw naw naw" (one nine nine nine).
Weeks are generally referred to by the date on which they start, referring to the Monday, eg "week commencing 5 March". ISO 8601 week numbers are found in diaries and are used in business.
British Standard
ISO 8601 has been adopted as British Standard BS ISO 8601:2004, and is popular in specialist use (eg use-by dates on medical products) and computer applications eg database systems, communication protocols, web pages, and so forth. The appearance of such big-endian dates is increasing especially with computer-generated forms and invoices, as well as simply internet-accessible content in an age of Globalisation.
English
Both the 12-hour and 24-hour notations are used in the United Kingdom. The 12-hour notation is still widely used in ordinary life, written communication and displays, and continues to be used in spoken language. The 24-hour notation is used in timetables and in some computer applications; computers running Microsoft Windows with UK regional settings default to display time in 24-hour notation. The 24-hour notation is used more often than in the United States especially for bus, train and airline timetables, but not quite as commonly as in much of the non-English speaking world.
You do not speak in 24-hour time (ie you would not say "twenty-one thirty"), instead you convert it into 12-hour (ie 21.30 would be pronounced "half past nine"). However, the spoken 24-hour system is sometimes used in airport/train station announcements: "We regret to inform that the fifteen hundred (15:00) service from Nottingham is running about 10 minutes late" / "The next train arriving at Platform 4 is the twenty fifteen service to London Euston".
To separate different parts of time, either a full stop or a colon can be used. A style which many people like to adopt is that for 12-hour time, a full stop is used, eg 1.45pm, whilst a colon is used for 24-hour time denotation, eg 13:45. Such formats are used by institutions such as the BBC, Evening Standard, etc; this is, however, not a definite rule - a full stop can be used for 24-hour time (the Guardian Style Guide recommends a full stop for both 12-hour and 24-hour time). A colon cannot be used for 12-hour time, eg 7:30pm would be incorrect.
You must not write eg 07:30pm, which makes no sense as you would be combining 12-hour and 24-hour time in a way in which the time you intended to write cannot be confidently decided upon as it is a contradictory statement. Writing eg 07.30am (or especially 07:30am) would also be incorrect, as the nought before the seven demonstrates that it is an am time.
In British English, the expression "half [hour]" is used colloquially to denote 30 minutes past the hour. For example, "half ten" means "10.30" (without specifying morning or night). This contrasts with the Dutch, German, Hungarian, Czech, Baltic and Scandinavian languages, where the same type of expression denotes 30 minutes before the hour.
Welsh
The Welsh language usage of the 12-hour and 24-hour clocks is similar to that of UK English above. However, the 24-hour notation is interesting in that it only has a written, not a spoken form. For example, written 09:00 and 21:00 are both said ("naw o'r gloch" nine o'clock, literally nine of the bell). Minutes are always either "wedi" (after) or "i" (to) the hour, eg 21.18 "deunaw (munud) wedi naw" (eighteen (minutes) past nine) and 21.42 "deunaw (munud) i ddeg" (eighteen (minutes) to ten). Phrases such as "y bore" ((of) the morning), "y prynhawn" ((of) the afternoon) and "yr hwyr" ((of) the evening) are used to distinguish times in 12-hour notation, much like Latin "am" and "pm", which are also in common use, eg 9.00yb (09.00) as opposed to 9.00yh (21.00).