Its no surprise to me online shooting supplies stores dont have everything in stock , naive to think they would have, think what it would cost carry dozens of each product across say 50 -60 manufacturers , Β£500k? Β£1m ? through in optics and guns and it would be several million , margins are low in any area of online sales so its not an attractive business model for many, its been mentioned "the biggest gun dealers " in the country have never got everything people order yet they probably carry more stock than anyone , that should tell you something.
What makes it less viable is when penny pinching types buy from outside the UK , they also do the most whining .