What’s the cheapest way to cash a US dollar cheque in the UK?

There is no cheap or timely way to clear a US cheque in the UK. Your bank is probably both nearly the cheapest and nearly the quickest.

You could mail the cheque back to the person or company that sent it to you, and ask for a wire transfer instead. Certainly if you intend to receive more money from them in the future, this is the way to go.

If you have someone in the USA that you trust you could sign the back of the cheque and send it to them, to be deposited into their account and then wire transferred to your bank account. While it appears that there are people who do this for people they just met on Facebook, I do not recommend reaching out in this fashion.

If you have a bank account in the US, you may find the cheapest way is to deposit the check to that account by mail, or from a phone, and then transfer it to your British account. Look up the transfer prices first. If you don't need the money immediately you don't even need to do the transfer- just wait until you next visit the States and take the money out then. Keep in mind that most checks go stale in 90 to 180 days, so don't hold the check for too long or you'll have to have it reissued, which could be extremely difficult.

Just a follow up as i found this thread while researching this, I have found Halifax do not charge a fee for cashing in Foreign Currency Cheques!