Just as an addition: "negotiation" means a faster form of getting money into your account, where the banks (US and UK) agree on a conversion sum, and the money appears in your account within a week or so.
I do not know the exact form this negotiation takes, nor whether the sum that appears in your account is available for withdrawal immediately.
"Collection" is a six to ten week process where the cheque is sent back physically to the US bank, and various time consuming procedures which I also don't understand are used to make the US bank actually wire the real final amount to the UK bank. In this case there is a huge delay but once the money is in, it's available for complete withdrawal instantly.
I know this because one of my offshore banks used to be able to cash US dollar cheques within a week by negotiation, but later changed to collection only with a 6-10 week delay.