It depends what you're doing, Gertrude. For a personal website, you don't care.
For something more serious, you want the right result even if folks miss off the .uk (or add it in error), and you don't want some virus-site pretending to be you. For £10 a year (3 additional registrations) it was worth it for what I was doing.
Sometimes, people phone and ask "has your website got a .uk on the end, or not?" I reply "Yes <pause> it will work the same with or without."