The best place is a debit card in an ATM in France.
Depending on your bank though there may not be a lot in it - most charge about 4.25% on the interbank rate in commission/charges. Nationwide are free.
If you get euros in the UK you will be paying 5 or 5.5% generally on the interbank rate. You won't know it - it will be hidden in the exchange rate and advertised as 'commission free'.
M&S tend to have the best rates - also if there is a Cheque Shop near you they are worth a try