The UK should not attack Syria or Syrian Government forces or facilities.

Following an agreement, the area of the alleged incident was being evacuated of rebels, their families and civilians, so it seems unlikely that this incident took place.
Even it it had then the UK should not add to the damage already done, largely by outsiders, to this unfortunate country over the last seven years. Any attack will tend to prolong the war and delay rather than advance possible reconciliation between the moderate rebels and those who support the Syrian Government.