Upvote:2
One possible duplicate has not (yet?) been answered and the other focuses on French citizenship and contains some information that seems inaccurate.
All the 'Indian' sites I have seen are of terrible quality and very confusing so I suggest relying on the unequivocal:
To transit through India you will need a transit visa.
from GOV.UK
with support from World Travel Guide:
For UK nationals: transit visa: £56
and
valid for three days of transit through India within 15 days of the date of issue.
At least at the moment you are not eligible for a visa-on-arrival arrangement.