American Airlines and United Airlines are offering huge wads of frequent flyer miles for travel to/from London (and, in the case of AA, to Manchester as well). Through June 2009, you can earn up to 50,000 bonus miles per roundtrip if you fly over the Atlantic in higher-priced booking classes.

The 50K bonus is just for paid first or business class (discounted Z fares count!), while 25K is for the upper-end coach fares.

You’ll find American’s offer here and United’s here. Registration is required for both, so be sure to follow the registration links on those pages if you want to qualify for the offer. Remember, also, that you have to be flying to or from the UK. Transiting the UK to another destination won’t count.

[While you're at it, make sure you're signed up for the double elite-qualifying mile bonus offers that American, United, Continental, and (as of yesterday) Delta are pitching. Registration is required.]

Why the sudden burst of big-ticket offers?

Oversupply, oversupply, oversupply. Obviously, the airlines are trying desperately to fill seats, and in the case of the UK offers, fill those seats at higher price points. UK routes are particularly common among business travelers, and the mauling of the premium-cabin market has to be taking a bite out of the airlines’ projected revenue streams.

Plus, they know Americans are hooked on frequent flyer bonuses, so they’re trying to keep you from moving your business to the loving embrace of Virgin Atlantic or British Airways, both of which are offering great deals to the British isles these days.

By offering bonuses like these, the airlines are effectively throwing in a free (restricted) domestic ticket when you buy a UK-bound seat at full price. But realize that it’s not necessarily a bargain.

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