You do need to book 25 nights (off the room block) to get the free room, but you also need to guarantee a minimum number of nights as you're a Wishes wedding anyway.
With 30-40 guests, and coming from the UK, you should be fine.
Right - the 25 Disney room nights is a requirement for having a Wishes wedding, whether or not you book them through the block and they earn you a free night.
It's like the overall minimum expenditure requirement and the per-person minimum expenditure requirement - these are rules Disney sets to make it worth their while to host weddings at WDW.
to qualify for the free night they need to book through the block. Disney will send out Save the Dates that have information on how to book, but basically there is a group reservations line specifically for weddings. They can't book through the block just by calling CRO. I'm not sure if a travel agent can book for them. I'm assuming they could make the call for them, but not sure how that works with the commission an agent would normally make.
They can book outside the block and that qualifies for the amount of rooms you guarantee (but not the free night), you'd need their reservation info and you just send it to your Reservations contact and it's added onto your list.
This has recently changed back to the way it used to be, so now any reservation at a Disney resort counts toward the free night again, even if it wasn't made through the Wedding Groups reservation line. As pezaddict said, you would just submit the info for people who booked other ways to your wedding consultant. However, if you signed your wedding contract under the previous rule regarding the free night, it will still apply.