FYI, to be able to run such a charter as a commercial venture, you will need ATOL cover. Whilst charter carriers may be able to give you a price indication, they're legally prohibited by the CAA from operating such flights unless the charterer has applicable ATOL cover.
In reality, you may find it necessary to contact a decent broker that will actually be able to cover you with ATOL.
Try:
www.chapman-freeborn.com or
www.airpartner.com which may be able to help further.
As you'll probably guess, ad-hoc specialists which frequently have aircraft available, such as Titan Airways and European Air Charter tend to be more expensive than other charter operators offering similar capacity aircraft, which generally have much more restricted availability.