I have a sum of around GBP100,000 that I need to transfer from the UK
to the US. I have been told that a wire transfer from my UK to US
checking accounts will not get a particularly good rate of exchange. 
What are my other options?

As an ex-banker, I don't think you have any options other than the
choice of having the UK bank transfer the funds in Sterling OR in US$.
 In the first case, then the US bank would handle the exchange to
dollars, in the latter, the UK bank would, which I would recommend,
since it will be more familiar with exchange.
You may ask the bank if it makes any difference  - what difference - 
if it exchanges the funds immediately OR if it makes a foreign
exchange contract that will be settled two business days later and
then transfers the dollars.
I rather doubt that it will agree to making a contract for that
amount, and it probably wouldn't make much difference.

But I may be wrong, and then someone will come along and say so  - I hope.
Myoarin