I guess the conclusion is that if you are using Mondo’s card abroad and you are only visiting the country for a few days then just pay by card. If you need cash, use the Mondo card in a local ATM and avoid Travelex / Post Office / et al (making sure to let Mondo/Mastercard do the conversion rather than the local ATM)
As for 1000£ you’ll get 1398€ with Mastercard’s rates but only 1345€ with Travelex (a 53€ fee!) or 1350€ with Post Office (a 48€ fee). [noting these are tested on the 9th at 3:30pm so different day than the aforementioned mastercard & transferwise rates]
This however doesn’t really apply to other banks or credit cards as they do have additional fees above mastercard’s rates. e.g Lloyds current account is 1.5% with min 2£ - max 4.50£ for cash withdrawals or 2.99% + 1£ on card transactions. Natwest/RBS is in a similar range, no doubt the others are too.
I used to use xe.com as the rates were better than the bank’s own but at 1000£ => 1368.9€ it is better than the retailers but no better than visa & mastercard’s rates & Transferwise is definetely a much better choice for those large 1000£+ transfers.
With regards to why having multi-currency account would be worth looking into for Mondo, currently Barclays and a few others do have Euro accounts but they used to charge exchange transfer fees from your euro balance when doing a euro transaction (WTF?!), it seems they’ve reworked the product but still have weird 25£ fees to do transfers from that euro account to another SEPA euro account or 6£ fee if you want to pay into it.
The only one which doesn’t is Citi with the Citigold account which allows GBP, USD & EUR balances tied to the same account & card without any fees https://www.citibank.co.uk/personal/citigold-current-accounts.do but account eligibility is really only for the elite (£75000 balance or 300£ annual fee).
There are a lot of European nationals in the UK/London so it’s not a small market. London is France’s sixth biggest city and if you add up all of the other EU national, that probably is a market of about a million people who could do with consolidating their various accounts into one.