Whats the best way to get a good exchange rate when transfering US dollars back to GBP?

Can anyone tell me the best way to get a good exchange rate when transferring approx $180,000 US dollars back into GBP as the banks are not giving a good rate but some company's over the internet are giving a far better rate. has anyone used any of these company's before as i am a bit wary of using them in case my money goes missing or they don't provide the exchange rate they are advertising