If your journey isn't too great a distance, for example you are visiting from Europe, you might want to consider a variety of travel options. But if your journey is a long one, plane fares are now so competitive that in terms of speed, comfort and cost you should probably just look at flights.Flying is most obvious way to travel to UK. Fares can vary dramatically so best advice is to shop around and be flexible about dates and airlines. In particular students, young travellers and seniors should be able to get special deals and discounts.
Prices are generally a bit lower if you travel during week. Peak season is from about May to October and this is when fares will be most pricey with most expensive months from June to September. Fares will probably also be very high (and flights booked out in advance) over Christmas and New Year.
The busiest and most popular airports are Heathrow and Gatwick, both in London. But if you are not visiting London you might try some of other airports in UK which handle international flights, such as Birmingham, Manchester, and Glasgow. You could always travel to UK by ship, for example you could choose a transatlantic crossing on a big cruise ship. This would mean that you arrive in UK in style and without hassle of jet lag, but any long distance journey by ship will be very lengthy and, of course, very expensive compared to flying.
Another way to get to UK by water is to travel by ferry. Crossings are very extensive and are a great way to travel to UK if you wish to bring your own car. Prices vary enormously as ferry fares can change according to time of year, time of day you want to travel and even size of your car - so you will need to so some careful research to find a good price. And remember that during peak season you will need to book a place for your car in advance.