Sentimental value, an added sense of national identity.
More practically, it allows the UK government to establish monetary policy specific to its needs. Greece is not able to do this to any degree, and its economy went into crisis with no way to help fund its ballooning debt. Germany, heavily dependent upon exports, would do well to be able to devalue its currency and adjust interest rates to be competitive in sales to the US, Canada and eastern EU countries. The Chinese, Canadians and Americans can and do, as do the Swiss. By slightly increasing or decreasing the interest rates and thus devaluing or increasing the value of its currency, a country can better hedge against inflation, and be more competitive in exports (lesser value), get more for imports (greater value) and attract foreign investment (greater value), depending on its particular need(s). And while there are studies that show costs lost in currency exchange, there are studies suggesting the services provided in changing and managing currencies negate much of these costs.
By the way, you cannot travel anywhere in the EU with the Euro. The Czech Republic, Romania, Bulgaria, Sweden, Denmark, Lithuania, Latvia, Estonia, Hungary, Poland and the UK all retain their own currency, while the EU is allowed and utilized around the big cities, prices are not in Koruny/Forint/Pounds and Euros everywhere, and in smaller towns and villages a Euro is not often used.
Britain does very well as a financial center using the value of the British Pound. They stand to lose some of this financial independence as the Euro's financial hub seems to resonate in Frankfurt. So converting to Euros may not be in the UK's interest, certainly not London's.
Not from the UK, I was aware of the fact that the Queen would be on the coin currency (as is Beatrix in the Netherland's coinage). I do care personally, as a former resident in the Euro zone (Czech, Italy) with many ties still and as an Economics student, I consider it an important question. I'm not convinced of its long term benefits in certain countries like Italy, whose fiscal house is not in order and won't be for decades without monetary manipulation of currency which it cannot do, or the Balkans, some of which have adopted the Euro with hopes of joining the EU but are in even worse shape than Italy and have unique problems entirely different from Germany's.
Also the experiment of joining such countries differing radically in their economies (Kosovo, Bosnia & Herzegovina, France and Germany all use the Euro) to one currency takes away financial independence. I'm not Euro-skeptic and very much support the EU. But a single currency has its limitations and it's like using the same tool for 27 different jobs - there are better tools custom built for each job.
Having the Euro when EU momentum is moving forward can be a plus (see Ireland, Spain).. not having it can also be a plus (see the UK, Czech Republic, Slovakia, Slovenia). But having it when the continent has stalled leaves a country with unique financial problems with its hands tied.