Just enjoy the experience of walking into a British Post Office and saying, with your chest all puffed out:
'I'd like to buy some US Dollars please'
Then listen to the assistant say:
'Yes and how many would you like madam?'
Then feel yourself go all weak at the knees as you ask:
'Do you need to see my passport or anything like that?'
And swoon when she says
'No madam,nothing at all, in fact you don't even have to sign anything.'
And leave the post office not feeling like a criminal and post it on FB.
This is what I did last December and she called me sir not madam.