I've just done a dummy booking for 14 July - out from Penzance to Tresco in the morning, back in the late afternoon - the website seems to be happy to do the booking and ask for my credit card.
£245 for a day trip for 1 person is a touch on the expensive side, but last year people seemed quite happy to pay a similiar price on the helicopter for Land's End-St Mary's - although I guess some people might have done the helicopter just one way for the novelty and travelled by fixed wing / ship in the other direction