On my last trip, I purchased Euros and Pound Sterling ahead of time. It was a very good feeling to arrive with local currency. I'll not be returning to UK until June, so I'll just need to purchase some Euros this time. The exchange rate is quite decent this week, so I will go this morning to order them at AAA. Last time I traveled, AAA had them for me within 24 hours. Both my husband and myself must use wheelchair assistance at airports, and it is important to tip in local currency.
Note: I received a call 17 hours later that the Euros were in.