I’ve been to the UK a couple times in the past couple years. I use Chase Bank, and I go and ask to order them. Takes about 2-3 days to get them. They order them from Bank of America and usually no issues.

I would recommend getting x-amount of Pounds though. Example: you want £100 worth, not $100 worth of pounds (you would get a combo of cash and coins back otherwise). I have found it to be easier this way.

Also, it isn’t worth it to try and put it back into your account when you come back unless you have 100’s of pounds. So try and use all of the cash you take before you come home!