I don't know about anyone else, but on one estate I am required to fill out a cull card for every deer that either I shoot or that a guest who I'm guiding shoots.
I have taken to using an iPhone app called GB Grid Ref Worker. It takes a GPS feed and converts the results into a 10 figure OS grid ref. It also allows you to take a photo (if required) and then you can send the result by email or text message.
I've no relationship with the developer but I have to say that I've found it very accurate and has saved me time faffing about with Streetmap. By taking the reading quickly it also helps remind me of time of shot, etc. and I can annotate the message with any relevant details (shot reaction, etc).
It's £1.99 which I've found to be money well spent. You can find it on iTunes: GB Grid Ref Worker on the App Store on iTunes