Foreign Accounts to Insulate Against Brexit?

You don't have to open an account in another country, you can hold euros or dollars with U.K. institutions.
You can also (of course) hold foreign investments, that's what you are doing when you hold most global funds, bonds and ETFs etc. (Unless they are hedged back to sterling)
You are though about 2 years too late and going against the grain, in that if most financial institutions thought the Pound would "crash massively" ina few weeks time that would happen right now, they wouldn't wait to the day, they would be bailing out now, indeed they would already have bailed out and the crash would already have happened.
Now you might if course be right but you are disagreeing with what global financial markets think will happen so there would need to be something additional since the U.K. leaving is already priced in. What isn't priced in for sure is the nuances of what leaving means, but that's unlikely to lead to a "massive" crash. The Pound has already taken a hit since the vote , and I think it's actually risen the last few days (just as you are thinking of getting out!)
Have you considered the downside, an agreement of any sort would definitely cause a rise in the Pound and thus a relative fall in all the money you've just shuffled into ?Euros? Dollars? Plus add in the losses on the transfers to and from sterling and the downside is certain and the upside not so much.
My view is any sensible person would already be globally invested across a wide range of types of investment so would have no need to make panicked last minute moves of currency and investments because trying to guess global currency markets and trade them as an amateur is a mugs game.

But if you want to do it, as I said to start, just open a Euro or dollar account with any of numerous financial institutions, buy funds that follows the Nasdaq, the S&P500, the DAX the Hang Seng etc etc etc and fill your boots.