Take a look at www.ryanair.com (for example) and you'll find lots and lots
of very cheap flights from London to other European destinations. They're
not the most customer-service-oriented airline in the world, their flights
often leave or arrive at inconvenient times, and their airports are often
a distance from the cities they creatively suggest that they serve - but
if you have a week, none of this should matter too much.

You can also get surprisingly cheap flights on more mainstream carriers,
but if you don't have any particular destination in mind, you might as
well just see what's lowest-priced on a budget carrier like Ryanair and go
with that.

Sometimes they do special-offer flights for a euro plus taxes (my brother
claims to have bought one for a *cent*), in which case it's not really the
end of the world, or the credit card, if you book and decide closer to the
time not to travel...

I would also add that if you are booking less than a month in advance, you
may find it cheaper to book with a 'proper' airline. BMI usually has cheap
fares to a number of destinations in Europe www.flybmi.com also check Air
France www.airfrance.com I've picked up a few last minute flights with them
at very reasonable prices.