i cannot say anything about rotorheads, but in our company we regurarly do ambulance missions with aircraft , and when it comes to fuel it depends not if its ambulance or not, it depends if you are as the company entitled from the tax authority to take commercial fuel which is tax and VAT free. at least at our german operator.
you have to show to the pump attendant the such called "erlaubnisschein" or a allowance ticket and then he will bill it tax and vat free.
there is also antother vat : the general VAT when you fill the invoice tor the entire flight. here it depends- flights which start and end in the country of registration have to be billed with VAT, international flights are generally VAT free.
but things in UK or generally in helicopter operations may be different .
best regards