Hmm, I'm afraid Americanas website won't ship to London (they say they only ship internationally CDs and Books and only to the US and Japan.)
But hey try CD Point. They do ship worldwide. The price is not as cheap as the other one but still is very cheap yet! R$16 (R means "Real" which is the brazilian currency), something around 10 dollars (shipping not included).
Here's a step-by-step:
1) go to www.cdpoint.com.br
2) get more information regarding international shipping by clicking on the INT'L ORDERS tab
3) to find the DVD, go to the search form (at left) where it's written "pesquisa Rápida", choose DVD (1st field), then, with "Artista" (2nd field) selected, type.... ha...oh, yes, Donna Summer (lol) (3rd field)
4) all the results will appear. The 2nd DVD titled "Donna Summer" is the one you're looking for. Confirm it really is it by clicking on the blue ball (written "detalhes"). A pop-up window will open.
5) back to the main page, just click on the shopping cart icon ("incluir no carrinho"). A "1" will appear on the checking box.
6) at the top of the page, there's a shopping cart icon which says "ver pedido". Click on it to start the buying process
7) you'll have to sign in
8) here's a translation of the things that will appear on the sign-in form:
Nome - name
Endereço - address
Numero - Complemento - Number - Complement (like apartment #, for instance)
Bairro - quarter
Cidade - City
Estado - State
CEP - Zip code
Confirme seu E-mail - Confirm your e-mail
Telefone fixo - home phone
Telefone Opcional - ptional phone
Then there's an optional field for CPF (it's a document that only residents in Brasil have)
Data de nascimento (optional too)
Identificação - your login screenname
Senha - Password
Then you click on "OK - Confirmar"
Well, I don't know what will happen after this point. I guess they'll ask you if you're going to pay by credit card or another way. Let me know if you need more help.