There are three exchange rates of interest when changing currency. Let's start with the main headline rate that is quoted by financial news outlets like Bloomberg or BFM Business(1). That is what I call the "centre rate", upon which all other rates for exchange between those currencies are based. The centre rate is the rate paid by forex traders who often deal in nine or ten figure sums per transaction.
So, the other two rates? They are the "buy" and "sell" rates. Let's talk about exchanging between dollars and Euros, since that's the sort of forex transaction I'm most likely to have in my everyday life. The "buy" rate is the rate I pay when I buy dollars with euros, and the "sell" rate is the rate I pay when I sell dollars for euros. These rates, in effect, are the centre rate modified by a notional commission rate in favour of the bank / bureau de change / Marks & Spencer(2). (This is the origin of the name "centre rate", since it lies in between the buy and sell rates. A bureau de change in a Eurozone country will normally fix their buy and sell rates for, say, Euros-vs-USDollars as the centre rate minus(buy) or plus(sell) some number of USD-cents per Euro.)
A lot of credit and debit cards will quote the centre rate when converting a transaction quoted in a foreign currency, and then add in an exchange fee. My (French) bank does this, and the fee is (a) small and (b) dependent on the amount of the transaction. I'd say that 11 quid is a bit steep by my standards, unless your DLL was priced somewhere over $1000.
The alternative is that they charge a "buy" rate, but that causes all sorts of problems, since some fraction of the less knowledgeable customers will see a discrepancy between the centre rate and the rate on their bill and kick up an unholy fuss.
(1) BFM Business is a French business/financial news organisation, available in various formats. It is similar in coverage to Bloomberg's news offerings, but with a French slant. BFM originally stood for "Business FM".
(2) I don't know about now, but up to 2009 (during which year I left the UK, and haven't been back since), M&S was the cheapest way to just buy/sell foreign currency against pounds, since their "spread" (difference between the buy/sell rates) was smaller than anyone else's. It was amusing watching Thomas Cook staff in the queue at the M&S exchange counter, as if they could get a better rate there than any putative staff discount...