Doesn't seem possible.
I mean - could you potentially get 50 pounds worth of physical British currency at the current exchange rate from a US bank? Certainly - but it won't be proof condition - not by a longshot.
I think if you wanted proof condition (uncirculated) for any note, you have to contact the treasury of that country - which I believe (and correct me if I am wrong) for the 50 pound note, is going to be the Bank of England (I've linked to it above in another reply).
In the US, I believe you have to contact the US Treasury Department; but you could probably just go to a bank for such a note (or an entire filled band if you wanted it) - but for certain things you would likely need to contact them directly I think.
For instance, you used to be able to get uncut bills; IIRC, one of the Steves of Apple fame used to pay things as a joke with these, in the one dollar denomination - he would get the sheets, then have them professionally perforated, and then punched to put them in a binder, and when he'd pay for stuff, he'd open the binder, and tear out the money - it was perfectly legitimate cash, but it threw a lot of people for a loop - I believe it was Woz who did this; Jobs was a bit uptight for that kind of hack - which is what made them a perfect pair - it's just too bad that Woz didn't get to really share in the "fame" of Apple (I think that was his choice, though - from what I know of him, that's how he is mostly).
To get those uncut bills, though, I think you had to travel to the US Treasury and purchase/pick-up in person; it wasn't something you could just easily grab at your bank. Today, they don't sell them at all, probably due to counterfeiting (bleach the paper and re-print higher-denomination bills all at one shot).
Still, I stand by my point that you can get a note of the local currency from a bank brand new (you may have to ask around) and it will be good enough for someone who just wants to have a new one and isn’t going to get it graded.