Well the code 0552/6645-99 denotes Royal Navy (0552) Military wristwatch (6645) and NATO code for UK (99) so it's not an Army watch for a kick off. It's also highly probable it has never been issued especially if it's pristine. An issued watch is more likely to be banged and scraped.
I have bought three of these mint G10's in the past few months, the highest serial I have is 45904 and the lowest is 29582. That's over 16000 watches between the serials of just the watches I have. I doubt the Navy issued 16000 watches in '89. A more probable explanation is it was an over order by the military, that very few were issued and most spent there lives in boxes in the stores. They have been getting shot of them at the Military disposal sales. There are tons of Royal Navy 89 G10's for sale, probably the easiest model to get hold of.
I don't know the seller but he's over egging the pudding in his advert :) !