The middle price is around $1.75 for £1 at the moment. The difference between that and $1.68 is made up of charges and shopping around can reduce that a little.
Unfortunately this is one of those situations where you will have to make up your own mind and live with the decision.
The $/£ exchange rate held fairly steady between January and the end of July. Since then it has fallen steadily (by about 10% in August) and, if I was to guess what will happen next, I would guess that, because it fell last week it will also fall next week. Such logic, of course, is a long way from infallible.