Whilst Nationwide may not make a foreign currency withdrawal charge (The Post Office credit card does not either) you are unlikely to be getting the best rate of exchange in the world. We have a French bank account and use UKFOREX Ltd. to transfer money from our UK bank account into the French one. We looked at other firms, but at the time we signed up with them they were offering the best rates.
Although we keep the UK cards for emergency withdrawals we would never choose this as our regular means of getting Euros. Neither would I recommend using your own UK bank's foreign currency service, as you will pay through the nose.
As a previous post suggests, it is best to buy currency in fairly large chunks to get the best value for money, so if you have enough Euros to tide you over, save up your pension payments in a UK interest-bearing account until you have a few thousand to transfer.
The way it works is that you phone the currency dealer and make the "buy" order, then pay the dealer in sterling from your UK account. If you have online banking this is fairly straightforward as you can set them up as a payee. The dealer will then pay the Euros into your French acount. The whole cycle takes a minimum of 3 or 4 days, not because it HAS to, but because that is the way the banks choose to operate. Whilst the funds are "in transit" the banks get to use them free and gratis for their own benefit.
I have a feeling that some people might be a bit intimidated by the idea of using a currency dealer, as it smacks of stockbrokers and day-traders and all the other City high-flyer stuff

which is out of their comfort zone, but it really isn't like that.
I assume that when Scattycat says their bank refuses to pay their pension into a foreign bank what they mean is that they cannot get the sterling automatically converted into Euros and transferred. I would be surprised if any UK bank would offer this facility, but I would not trust them not to rip me off if they did!