Forget about Land's End to John O'Groats the real northern end of the UK is Muckle Flugga*. It's unlikely that you'll ever get to land on Muckle Flugga, it's a small offshore stack with a lighthouse on, but the RSPB reserve at Hermaness gives wonderful views over the Gannet colony there .... and to get to there you will have traveled across Shetland and seen some wild and beautiful islands so take your time and enjoy the trip. Information about the ferries to the various islands that make up Shetland can be found here
.*To be precise there is a small rock, Out Stack, just a little bit further north but it doesn't sound as good as Muckle Flugga when you say it.