You'll not need to inform them beforehand, but you will need to ensure that you get the correct information to prove that you made the attempt, so something that conclusively proves that you were at all stations when you said you would be.mrredrunner100 wrote: Does anyone know what happens if this happens because me and my friend have been wondering about this?
Various challengers use different methods: log books charting arrival and departure time with train numbers, time and date stamped photos, videos with the day's newspaper and you arriving/departing a location, witness statements, and so on. You'll need to submit this all to Guinness to prove that the record actually happened (and be prepared for them to contact you for further information to verify the claim!)
You'd also be wise to get an independent person to verify the start time and finish time (eg have someone else with you when you start and end). Essentially, if you can't prove that you got the record to the degree that Guinness want, they won't be able to verify your attempt. Bizarrely they also don't recognise attempts made on a weekend, but there are a couple of historical threads about that as well!
It might also be worth bearing in mind that the Met Line currently operates a Leaf Fall timetable, so trains are slower and take longer (and depart earlier).
From previous experience of challenging in late December, you will come across christmas shoppers and drunken revellers, all whom might cause delays!
Best of luck - let us know how it goes!