We're planning to do a lot of locks this trip, so we thought it might be an idea to have some form of communication between the helmsman and the lock worker. We've seen other boaters with two-way radios, and they can be really useful when the next lock is through a bridge or round a corner.
So I ordered these from Amazon, and even with the free postage option selected, they arrived in just a couple of days. They were only £15 (paid for with some Amazon vouchers I'd earned for doing surveys), and they certainly work from outside the back gate to the kitchen. It'll be interesting to see how much we use them.