My Samsung Galaxy Portal runs Android 2.1. I could find no documentation explaining how to enter a “£” British UK Pound sign (and other symbols) into messages.
To display a £ symbol in a message, on the Android pop-up keyboard, simply press and hold the $ symbol. Another pop-up appears showing a number of currency symbols, including cent, pound, euro, yen, and others.
Pressing and holding other keyboard symbol often brings up other symbols too. eg. pressing and holding the round bracket, displays square and curly brackets.
I subsequently found that the Android 2.2 User Guide has some notes on this in the section “To enter numbers, symbols, and other characters” on page 32.