Balance of Power

The British policy in the Continent was to ensure no single power dominated Europe. This would thus ensure (in theory) that no one power would be able to turn its energies to rival Britain at sea. After 1815 the remaining Great powers were Russia, Prussia, Austria, France, and of course Britain. (I leave out the Turks, Sweden etc as they are secondary to this).

Now, with France toppled the new buffer states created by Napoleon had to be removed, but the problem was to keep one power from dominating that area. Additionally each Coalition member expected some territorial reward for their efforts.

Distribution

One could not create a new Power out of the Napoleonic territory given to Prussia as no single nation had a concrete claim to that territory. Giving it to Austria risked Austrian hegemony over Central Europe, Russia was not interested in Rhenish land, and France was ofc out of the question. Thus Prussia received a vast swath of territory on the banks of the Rhine, which was historically a very different landscape politically and economically from East-Elbian Prussia. It was intended that Prussia act as a 'gate-watch' for Germany in the event of French revanchism. The territory which the Prussians really wanted was Saxony, and that was split in half with only half going to Prussia.

Thus, Britain gave Prussia land which would prove more difficult to govern than and geographically non-contiguous with Prussia as a whole. The land they really wanted, Saxony was given only in part and they lost some of the Polish terrotory gained in the prewar Partition. It really didn't make Prussia great automatically, and the Rhineland would prove to be a source of political unrest post-1848.

Sources - 1) Iron Kingdom by Christopher Clark is a good single-volume history of Prussia

2. The Rise and Fall of the Great Powers, Paul Kennedy talks about the 'balance of power' system favored by Britain