Adding Insulation to Exterior Walls
If it turns out that you cannot insulate a cavity, it is possible to insulate either the inside or the outside of the walls.

If you insulate the inside then:


your walls will be "cold", this means that the materials in them will have to survive in whatever conditions you have outside, with much less additional heating from inside the house.
you will lose internal space
you will have to redecorate, move skirting boards, etc.
your windows will be recessed more on the inside.
this kind of insulation will leave cold bridges for example between the floors and where the joists meet the walls, etc.


If you insulate the outside then:


the walls will run "warm" so the materials will be kept near the temperature of the house
the walls remain part of the thermal mass of the heated space which may help you maintain a more consistent temperature inside the house, instead of it getting hot quickly when the heating is on and cool quickly when it is off.
you will have to move any external fixings (drain pipes etc) away from the wall by the amount of the new insulation layer.
your windows will be recessed more on the outside.
the external appearance of your house will be altered (although there are many finishes available, including brick veneers). 
good at eliminating most cold bridges as you can cover the whole surface of the house in a continuous layer (apart from penetration by pipes, windows, etc).


In addition to thinking about how much insulation you will need and where to put it, you should also consider moisture control. Adding additional insulation anywhere will move the dew point (the temperature at which the water in the air in your house will condense), this could lead to condensation forming within the wall (interstitial condensation) and may require a vapour barrier, more/less insulation, additional ventilation, or for you to ensure that the wall can "breath" so that it can dry out when the conditions alter.