Using the Pound Sign
The pound sign is a capital L with a crossbar, inherited from the Latin word libra, the scales used to weigh a pound of silver. It denotes pound sterling in the United Kingdom and its Crown dependencies, and the same character also serves the Egyptian, Gibraltar, Falkland Islands, Saint Helena, Syrian and Lebanese pounds, which is why the ISO code GBP matters whenever the context is not obvious.
Placement and spacing
Sterling always takes the sign before the number with no space: £5, £2.50, £1,000,000. Pence written on their own use a lowercase p directly after the figure, so a chocolate bar costs 85p, never £0.85p. Mixing the two, as in £1.20p, is a common shop-sign mistake that editors flag immediately. In accounting tables the code replaces the sign, and both should never appear together; GBP 1,200 or £1,200, not GBP £1,200.
One bar or two?
Unicode keeps a separate character, the lira sign ₤ with two horizontal strokes, but the Bank of England and every British style guide use the single-bar form. Some older typefaces draw £ with two bars purely as a design choice, and that is fine, because the underlying character is still U+00A3. What you should not do is type ₤ deliberately for sterling, since it is a different code point and will not match a search for £.
Why Americans call # the pound sign
In the United States # was used on scales and invoices to mean pounds in weight, so the key on a telephone acquired the name "pound key". British speakers call the same key "hash", and the British £ key sits where Americans have #, on Shift + 3. That swap is the reason emails from a US keyboard sometimes arrive with # where a UK writer expected £, and the reason a UK keyboard in Windows shows a different character than the keycap when the layout is set to US.
Rendering and keyboards
The sign is part of Latin-1, so it displays correctly on every phone, console, watch face and web page without any special font, and it survives copy-paste into games and usernames that reject fancier Unicode. On Windows the numeric keypad code is Alt + 0163; on a US Mac it is Option + 3; on a UK Mac it is simply Shift + 3. iPhones with the British keyboard put £ where the US layout has $, and every other layout shows it in the long-press pop-up on the dollar key.
People add £ to bios to signal a shop, a commission price list or a UK-based account, sometimes alongside the euro sign for international audiences. Because it is a plain Latin-1 character it also converts cleanly through the bold fonts generator on this site, unlike newer signs such as the rupee sign that many stylised alphabets lack.