Then again. If you are playing against a friend. Just play with the cavalry reserve on the table and available. You have after all, bought the rules :)

May people in our group play with reserve cav brigades and don't seem to have an issue. Mounted units move far so it's not as bad as you may think, but part of the fun is working with what you have to get the best results.

Brunswickers get a light Cav in their brigade

you have to play using the best tactics available, take full advantage of the terrain, entrench whatever it takes to hold your position. But agree if you cant get reserve cav onto the table it can cause a headache, but nothing is impossible if the plan is thought through properly.

Now I know the british peninsula was moslty an infantry army.

But they don't have any Organic cav brigades, so if you play a one suport force game.
And as British light division, you have to survice for quite some time(mabye as an attacker) with out any cav suport and you don't know when it will arrive.

The other option is to go for an all infantry army were you get 4 more infantry battalions, but then you fight with out any cav for the whole game.

So how would that play, you fight the french, and they might have 4 light cav regiments from the start. won't the british just end up in square for the whole game, and get shot to hell by close range cannon that the french can bring up with out any infantry to stop them?