Maybe not wise to assume that this man was present until you have done some thorough research, as some men must have been held in reserve. A regimental history may hold the answer, unless we have someone who has a good knowledge of this battle.
I am reading the Spion Kop section of The War in South Africa available on this website and it speaks of eight companies being involved. I'm not certain, but I do believe that the Lancashire Fusiliers had a total of eight companies, so if this is correct, it may well indicate that all companies were involved in the battle.
Welcome to the Anglo-Boer War forum. I have checked on your man on the medal and award roll (WO100/179) on Ancestry and it confirms that Private 2895 R. Bicknell of the 2nd Battalion Lancashire Fusiliers was issued the QSA with the Tugela Heights and Relief of Ladysmith clasps. The roll also makes mention in the remarks column, that he was Invalided on 04/05/00. Roll as follow -
I have also looked on the 'Boer War Casualty Roll' and I haven't been able to find any other information with regards to the reasoning behind him being Invalided.
There you go Wayne, I think Trev has solved this. I think that this QSA clasp entitlement plus what is recorded about the eight companies of Lancashire Fusiliers backs up what you initially thought about this guy being present at the Battle of Spion Kop.