It would be a lot easier if people did a lot of things.
You are running a business that is based on convenience. People don't know where the nearest exchange is, they don't have time, they are here on meetings, have limited time where as you have lots of time. Not your problem, but if you are happy to lose work because you won't accept other currencies.. Pretty much anyone in a cash-based business would accept other currencies (that can be exchanged in the UK, not currencies that are hard to exchange), why create a negative barrier with someone that is wanting to give you money and business? You lose money via credit card transaction fees if you offer credit card payments, you get charged inward fees for international transfers from clients. All businesses have these expenses. The bank will exchange it for free or a small fee when you next do your banking.
It's all as valid as a pound. Just round up to the nearest 50 or 100. If you are charging someone 250 for some services and the exchange is 409, make it 450. If it's 470 just say 500.
Fake notes are very easy to spot. They generally aren't spotted as people are rushing and not looking (when they are used commercially). The paper never feels the same, even the really good ones.
Never had a single person argue when I say to round it up to the 50 or 100 (people understand there is currency conversion fees, and you are doing them a favour).