Accepting unconditional offerWatch
The earliest deadline for firming an offer is early May - there's no advantage to firming early.
If you're sure that you're happy with your choice then wait until April to put that decision into Track...if you're sure you wont change your mind and if you *do* change your mind then you aren't locked in to a decision.
Once you firm an unconditional choice then there's no simple way to change that. You can't change your mind and swap to another offer - the only option if you change your mind is to wait until July and ask to be placed into Clearing...or to take a gap year.