Interesting though this discussion is there are a few factors not mentioned.
Firstly the physical sign in the OP is quite small, to post some of the wordy suggestions on such a sign would result in it being so cluttered that the impact would be lost - the fewer words the bigger the impact.
Secondly as has been pointed out elsewhere RRB from Peterborough would require circa 10 road vehicles for one scheduled train, these would then take around 2 hours 20 mins to get from Peterborough to Kings Cross on a good day and vice versa, resulting in most drivers being unable to do more than one round trip including a break in Central London. Leaving aside the sheer number of coaches and drivers required to do this over the operating day, there isn't now the physical space at either Peterborough or Kings Cross to allow for that number of vehicles to be on site or in transition between journeys.
Thirdly Peterborough is not a terminus station in the normal sense, even enlarged as it is now i doubt that LNER/NR would be able to run a full service in each direction leading to fewer and slower services to/from stations between York/Doncaster and Peterborough
So to go back to Doncaster leisure travellers, leaving at 0900 give or take a few minutes via LNER/RRB would give an arrival at Kings Cross of circa 1300 at best, not leaving much of the day to do anything, and with the prospect of a rerun to get home. The message is clearly don't travel whether your at Doncaster or any LNER station further south, and those very few people that have to travel will always find a way.