I have returned from Poland with surplus Zloty and wish to change them into Euro before a trip to France.
Can I switch easily in UK - what / where is most cost effective method?
The most cost effective method is switching a minimum amount of times. If you're planning on spending more Euros than you currently have in France, it would be most effective to exchange in France. If not, your local bank is the easiest and likely even the cheapest way to to exchange.
However, if your money is in a Polish bank account (as asked in the comments), the cheapest conversion is by bank transfer. Banks usually charge less than manned exchange places. Although when your local bank is the one you have an account at, it will probably be the same.