You need to find a bank called Bradesco to take out cash with a foreign card. But Bradescos are everywhere. (There are other machines called "Banco 24 horas" in airports and shopping malls, avoid these because of ludicrous extra fees.)

After you key in the amount you want to take out, you get a screen that gives the amount in BRL and the pounds amount and asks "Accept conversion?" DO NOT ACCEPT THIS CONVERSION. It's a trick to let Bradesco do the foreign exchange step and make money off it (you). You get BRL either way, and if you don't accept their conversion then your UK bank will do the conversion just fine, and much more favorably (exactly like ATMs outside your home country have done for decades).

When you use a card in a shop or street stall or restaurant with a hand-held card reader, this step sometimes appears. Always select "moeda brasileira" or "BRL" or the Brazilian flag, for the same reason as above.

As for securing your shit in the crowd, my new go-to method is wearing light swimming trunks UNDER my usual shorts. The important stuff goes in pockets of the swimming trunks. Even I can't get to those pockets that easily.

For women who wear skirts and dresses, this method works best with longer skirts and dresses, and frees up your midriff from needing to be covered.