Hi Jan,
I did this tour last April and found that it was very easy to exchange Sterling whilst I was there. I would always hesitate to contradict Julia, but there is no need to take USD to exchange into Dinar; you will be able to exchange Sterling just as easily as USD or Euros, and at a better rate than you will get at home.
I notice that JY have changed the hotel in Amman from last year, otherwise I could have given you directions to a very good general store where you can exchange currency. However, I'm sure that your TM or local guide will be able to advise you whilst you're in Amman.
There is also a very good bureaux de change at Petra in the Petra Visitor Centre. The visitor centre consists of a number of shops and restaurants built around a central square. As you exit the square at the far left hand corner on your way to the Petra site, you will see the bureaux de change ahead of you on your right hand side.
So to summarise, if you want to take some Dinar from UK to get started that's fine but not essential, and you'll easily exchange Sterling while you're there. I personally took enough Dinar to last a couple of days and then exchanged Sterling in Amman and Petra.
Hope this helps and have a great time
Mike