Is it more economical, when traveling to England on vacation, to buy
British Pounds before leaving the United States (in either cash or
traveler's checks) or to purchase British Pounds in England using U.S.
Dollars or U.S. Dollar traveler's checks?

If you're not talking about a major sum but just need the usual handy
cash to get along from day to day, you can simply wait until you get
to England and then walk up to any bank ATM machine and use your
American bank account debit card to withdraw various amounts of money.
 The ATM machine will give you British Pounds.  It also means you'll
not be carrying a major amount of cash at any one time.  And it means
you aren't left with Pounds to change back into US Dollars when you
get back home again.  I believe your bank account, next time you get a
statement, will have added a 1% to 2% charge to each of those ATM
withdrawal amounts.  You can get the exact figure by asking at the
bank before you head to Britain.

There won't be much difference, except for the exchange rate. The
American dollar is pretty weak now so I might wait as long as possible
because there are signs the economy is coming back and it might
improve. My guess is that you would probably get a better exchange
rate in England, because availability of the pound might not be as
high at your local bank (depending on where it is). If you have the
time, shop around. There is no perfect answer for this. The best you
can hope for is a researcher with personal experience, and even then,
your experience may differ.

I have travelled a lot, and have stopped worrying about the matter.  I
buy foreign currency where it is most convenient, at the airport
before I fly, where one usually has time on one's hands, and if not,
then at an ATM machine, which is definitely a better exchange rate
than that for cash dollars, including the card charge mentioned.
There is a good reason for this.  Foreign currency is just worthless
paper for an exchange booth until it can resell it, no interest, risk
that it will decline in value the next day or two, whereas the ATM
transaction results in immediate payment to the system by bank
transfer.

For this reason, I tend to disagree with Tisme's theory:  the exchange
rate should be more favorable when buying foreign currency in your own
country, but really, the whole question is rather like that about how
far it is sensible to drive to buy gas cheaper.

Thanks for your comments.  I also have traveled quite a bit, but am
going on this trip with a Brit who is insisting that buying Pound
Travelers Checks in the U.S. is a far better deal.  I tend to
disagree, but thought I'd check it out with folks who might have more
experience recently in Britain.  I'll probably stick with the ATM,
since my bank has a deal with Barclays.  Much obliged for your
thoughts.