Quote (from the website of the Egyptian consulate):
"Nationals from UK , EU and USA travelling to Sharm El Sheikh, Dahab, Nuweiba and Taba resorts ONLY, for a maximum of 14 DAYS, do not require a visa prior to travelling as a free entry permission stamp will be granted upon arrival. If you intend to travel outside of the above mentioned areas you MUST obtain a Visa"
If in doubt, get a visa. A single trip visa costs £15 but a multiple trip visa costs just £3 more, at £18. Full details are here:
http://www.egyptianconsulate.co.uk/Visas.php
Chris