The Lithium batteries that are allowed in carry on luggage are specified as being spares for portable electronic devices or portable medical equipment:
https://www.caa.co.uk/Passengers/Before-you-fly/Baggage/Items-that-are-allowed-in-baggage/
A Lion motorcycle battery isn't in those categories and so could be refused to be carried.
You might be lucky but I wouldn't risk trying to take it on the plane.
However you could pack it in your toolbox transported by road by the trackday organisers.
The problem is that he dropped the bike today...