Some really useful perspectives - thank you all! It's good to read because I don't see the garage as a necessity, but clearly thus isn't going to be the case for all future buyers.
In terms of parking, we have an unusually large drive which can fit 6-8 cars depending how they're parked.
Storage wise, we have 2 decent sized sheds (room for another, or a brick out-building if needed by a future buyer, but not needed for us in the here & now). The inner third of the garage that we would keep as a storage area has a side door that opens on to a side passage, which runs from the driveway to the back garden. Would be big enough for bikes, but not kayaks or other larger leisure items. Our plan would be to put some decent shelving etc in there, hang bikes on wall and get as much out of the space as possible.
In terms of reception room size, we do have a large sitting room leading to a living kitchen, quite open plan, which can sometimes feel a bit "too" open as it's a large space. We don't necessarily NEED another reception room, but can see a benefit in having a dedicated smaller space for the children to play - then in future a snug or home office. Certainly not a necessity though.
Bedroom-wise we have 4 doubles. No loft conversion and no plans to add one, but lots of additional storage up there.
Interesting to know we'd need to spend a lot more than £15k, which is a bit of a deal breaker - although we have other building work planned (rendering the facade, adding a porch, changes to the rear extension), so could possibly negotiate a better price if doing all work at the same time, maybe...?