It is worthwhile getting them before you leave. For adults they are £2 cheaper than on the gate.
It doesn't indicate there is a deadline for buying on line, so could be worth waiting until you are nearly ready to leave or potentially buying when in their car park.
Im going to this, grabbed my tickets from their website. They don't let you save the tickets to your iPhone apple wallet like Welling tickets, so print off a paper copy of the ticket to be safe.
Be interesting to see how many other Welling fans will make the trip, anyone here going?