I've no idea what "the rugby" is but as you've mentioned Sky, sign up for a Sport pass on Now & then immediately cancel it & you'll only be charged for one month. Unfortunately you've just missed an offer of £9.99 for one month, full price is £33.99.
I’ve been trying to do the same thing. You should be able to switch Big Sport pack and then change every 30 days. Log into MyBT and see your options.
Had to opt for a 1 day now TV ticket as it seems you cannot turn Big Sport on and off. It kept steering me towards a new 24 month contract.
"The Rugby" is the internationals being played over the coming weekends. Only on Sky.