I've used Nationwide (my own bank for 25 yrs) in the past, and agree the charges are exchange rates are very competitive. However, they don't have the correct licences to transfer the money themselves, and have to use HSBC as an intermediary.

Several years ago this caused me a major problem as somewhere between Nationwide and HSBC they lost 45000 Euro's of my money It was eventually sorted out, but took nearly 3 weeks. However, there was a plus side, the exchange rate had fluctuated in my favour during this time and I was around £200 better off by the time the money went through