Due to the large amount of money involved, I looked for a mainstream foreign exchange company.
I first tried http://www.currencies.co.uk/ but found their vetting procedure overly onerous so gave up.
I then tried http://www.moneycorp.com/uk/ and found them very easy to work with and got a good exchange rate. Subsequent transactions for smaller amounts gave a poor exchange rate so I looked for a different company.
I then used Azimo who have always given great exchange rates but with the occasional minor problem with monies transferred not equalling the agreed deal.
Others have recommended Transferwise and I will probably use them next time given the problems encountered with Azimo.
I guess a lot has to do with exchange rate v risk appetite. Smaller companies will give the best exchange rate.