Westminister Abbey and Tower of big Ben
I traveled to Great Britain and Republic of Ireland for a short trip, in November last year. The tickets are cheap that time of the year and I had some pending vacation I wanted to use. I was visiting my cousin who lives in London and she had helped me get the VISA for the UK. The good news is that the UK VISA is accepted in the Republic of Ireland, for citizens of some countries like the BRIC ones, so you don't need a separate VISA for Southern Ireland.
In England, I spent about 4 days exploring central London, with one of my cousins, and did a day trip, by myself, to Stonehenge and Bath.