I guess the first question is 'how confidnet are you with the bty numbers?
WO212/494 first entry for 102 ATk (ie their arrival overseas) states:
6/4/41 (Less A Bty left in Egypt), 287,288, 289 Btys, 1 Aust Corps, Lustre Greece).
The only other entry for the regt to mention a bty is;
19/8/42 (Less 287 Bty) GHQ (BTE Area)
However, 102 were only converted from NH in Feb 40, and initially in UK seem to have been the LAA/ATk Regt in 2 Armd Div. Given this conversion it seems reasonable that they started off with lettered btys, basically converting sqns into btys. Conversion from LAA/ATk to ATk could mean they did a bty swap with another regt, although I'd have thought that being in UK conversion could well have been the go ( In 1944 when LAA/ATk regts were formed in India/Burma they did bty swaps, but when they converted back to atk they converted btys).
99 doesn't seem a very likely number for a TA atk bty.