I have seen questions on the Helpline about getting Euros in London before a trip to Europe and the answer has always been that for the best fees and costs wait until in Europe and go to an ATM there to withdraw the Euros.
There is a new plan which has been running for a couple of weeks - full disclosure that I haven't tried it yet - where 70 Tube stations are getting or have now got ATMs which dispense both Pound Sterling notes and Euro notes. These ATMs are being provided by Raphaels Bank in association with Transport for London. The bumf that I saw said that "withdrawals of Sterling incur no charge while Euros come commission free". I note that it very carefully says nothing about not making a withdrawal charge fro Euros, nor does it mention the exchange rate but as TfL is putting these in as a service I would expect it to be quite close to the interbank rate.
The first ones are at major hubs such as Victoria, Kings Cross and Waterloo.
St Pancras station has had Raphaels Bank ATMs that I have used for a long time with no issues, for sterling only. I don't know if the plan for those is to make them dual currency.
If anybody has had experience using these new machines can they report back?