London is the one of must to see places in the world. It has long history and always been an important economic center. Well, if you landed to that page you're already consider trip here.
UK is an island, hence there are not so many options to get here. I'm sure you are going to take a fight to London. Other options do exist but they either time consuming or quite expensive
Here is a good news: London has 5-6 airports(technically only two, other are just outside of London metropolitan area). These airports accommodate enormous flights all over the world reaching pretty much all possible countries on all continents. As a hub for many low cost carriers such as Ryanair, easyjet, Norwegian, etc it is possible to find tickets as little as £5-10+ for European destinations and £150+ for North America