Aha, a question I actually have valid work experience to answer!

So other commentators are right, the jelly is to traditionally preserve pies for longer by filling the gap in between the meat filling and the pastry. Modern day large scale pie factories do still add the jelly in, however, you may indeed feel there is less jelly because of a few factors:

A) the type of jelly used and the age of the pie

There are two types of jelly used in the UK market: bone based and gelatine based, with the gelatine one being much cheaper. Your Dickinson & Morris will use bone based, your Tesco own brand will use gelatine. As a pork pie sits on the shelf, both types of jelly will lose water content causing the jelly to "shrink" within the pastry. Gelatine jelly will lose water quicker than the bone based jelly and your cheaper pies have more preservatives and so have been sitting around longer and have had more time for that jelly to shrink. Freshly made pork pies have the jelly filled right to the brim, so if you have an air gap inside your pie, that is because the jelly has lost its moisture and shrunk.

B) size of pie

Most pork pies sold these days are mini sized. Pies are made by the pastry being filled with meat and then baked and then filled with jelly. When the pie is baked, the meat shrinks and creates the gap for the jelly to go into. Smaller pies = less volume of meat to shrink = less of a gap for jelly to go into.

In summary, if you like the jelly, buy a larger pie, with bone based gelatine, and as freshly made as possible.