You don't patent it. Patents are for high end technological inventions, not slogans.
You want a trademark. This is done either in the UK, or it can be registered at the OHIM office in Alicante, Spain- this will give you a European Community wide trademark.
If you want to go global, this costs even more money. You would have to go to each individual country that you intend to do business in and register it there. I say 'do business in', because in the UK (at least) there is a ground for revocation based on non-use over a period of time. That's why trademarking slogans you won't use isn't a good plan- that and the thousands it would cost.
Alternatively, do what most businesses do and don't trademark it (saves a small company valuable funds). When you start to use it, you most likely begin to generate 'goodwill' over the slogan that customers and the general public come to recognise as your business tagline. With this, one can then protect through an action called 'passing off'. This is the unregistered trademark law, but can be harder to prove owing to the requirement of goodwill (hence why people still register trademarks).
Hope that windbag of answer sorts your question out!